There are several possible mechanisms going on here. I will explain them. If anyone wants sources I can cite them if necessary.
Most amphetamines and amphetamine derivatives have a very distinct sweet yet bitter taste. It's a very odd taste and it's rather repulsive (in my opinion). Amphetamines are also very well-known for causing your mouth to become very dry (cotton mouth) as amphetamines cause the body to produce less saliva. This is caused by alpha-adrenergic agonism caused by the massive Norepinephrine release of amphetamines. This can easily lead to odd flavor sensations in your mouth.
Conversely, amphetamine causes greatly heightened levels of ACh (acetylcholine) as well. This can actually cause the opposite reaction in some people as ACh is directly responsible for stimulating saliva production. I notice that I experience a sweet taste in my mouth when I take Cholinergic drugs as well. My mouth tends to get really wet (almost to the point of being unpleasant to be honest) and saliva naturally tastes slightly sweet.
This one is less likely, but possible. If you also notice a fruity smell coming from your mouth, this could be caused by Glucose release. I remember reading a study that was exploring the possibility that amphetamine could be damaging to people who are diabetic. In their study they found that amphetamine induces the liver to release stored Glucose reserves and therefore heightens glucose levels to some extent. Because Glucose is a sugar, and a very small percentage of Glucose can exit the body through exhalation, this produces a smell that is rather similar to Apples.
Apples (and all fruits in general, but specifically apples) have a very high Glucose content. What is mostly responsible for the smell of apples is glucose. When your breath smells fruity, you are actually smelling glucose that has been excreted by your lungs. Also, I forgot to mention that since Glucose in a sugar and more of it is now circulating in your body, it will automatically be excreted into your saliva as well. This is what's responsible for the slightly sweet taste of saliva and since Glucose levels are heightened by amphetamine, more glucose can enter your mouth through the salivary glands and produce a stronger sweet taste in your mouth.
If you're diabetic, another possibility is Acetone...
In the same study mentioned above - their results found that this heightening of Glucose that is caused by amphetamine, could be toxic in people who are diabetic for a couple reasons. First reason is due to the fact that Glucose can be hepatotoxic (toxic to the liver) and Nephrotoxicity (toxic to the kidneys) in high doses - this leads to Acetone.
People who are diabetic are already resistant to insulin, or if they are diabetic from birth, do not produce insulin at all. Insulin allows cells in the body to absorb glucose and use it as energy. If you have no insulin at all (in the case of people who are diabetic from birth - also known as type 1 diabetes) have to inject themselves with insulin daily due to the fact that their body doesn't produce insulin at all. Since there's no insulin in their bodies, their cells can't absorb glucose. This means the cells will receive nothing to breakdown and use as energy and would therefore cause death pretty quickly.
That's an overview of diabetes, but no I'll explain Ketoacidosis. When glucose can't be absorbed by cells, glucose accumulates in the blood. The brain recognizes that something is wrong and since Glucose isn't providing energy, the brain assumes that there must be no glucose in the blood. So it tells the liver to increase glucose production. This overwhelms the kidneys with glucose, but it overwhelms them with Ketone.
The body has a system to deal with these very high levels of glucose. That is ketone. Ketone bodies (there are three to be exact) are metabolites of glucose (and most sugars found in the human-body, the next most common being fructose) and they're a slight metabolite of Gamma-aminobutric acid - a very abundant amino acid in the body that serves many functions, but that's not really that important.
Anyways, this post become extremely long and I realized no one would care anyways so I'll cut to the chase. Glucose is metabolized into ketone bodies which are then metabolized and eventually broken down into acetone. When glucose levels are high in the body, much more acetone is produced as a by product of glucose metabolism. Acetone is toxic and is responsible for neurotoxicity associated with high glucose as well as responsible for hepatotoxicity and Nephrotoxicity.
Acetone can also enter the lungs can be excreted through exhalation. Acetone has a sweet smell thus accounting for the question asked above.
