You could be experiencing mild precipitated withdrawals. A common misconception (sometimes spun by medical professionals even) is that the naloxone in suboxone is what causes precipitated withdrawal when taken too soon after the last full dosage of a full agonist opiate. While naloxone certainly can cause precipitated withdrawals by itself, when in combination with buprenorphine, it is actually the bupe causing this, as buprenorphine has a much higher affinity to our opiate receptors than naloxone does. Basically, naloxone is an inactive ingredient that R&B use as a scare tactic to try and deter abuse, and IMO to keep their patent on this formula of buprenorphine now that subutex has become a generic medication.
So basically buprenorphine, being a partial agonist, can act like an antagonist if taken too soon in an opiate Dependant person. Like naloxone, naltrexone or any other antagonist, it rips the full agonist off of the receptor sites causing a very violent onset of withdrawal. However, these symptoms do subside as the buprenorphine begins to fill the receptors which had previously been occupied by the full agonist.
It is often recomended that someone transitioning from a full agonist to bupe wait at least 24 hours, or until they are in moderate to sever withdrawal. Seeing as you took hydrocodone only four hours prior to the buprenorphine, it is possible that you are experiencing what sounds like a mild case of precipitated withdrawal. For this reason I would suggest trying to wait it out a little longer. Bupe, like methadone can take quite a while to kick in. Taking more subutex may help, but by the time the additional dose kicks in, I'd imagine you'd already be feeling the effects from the .5 mg you took.
All that being said, you do have a tolerance to opiates, and it's possible that you will need more than .5mg of subutex to feel relief. You may want to start off with a slightly higher dosage, say 2 mg, and then gradually taper down from there. The good thing about buprenorphine is that it's relatively easy to lower your dose as long as you do it gradually.
ONE MORE EDIT//
Just to expand on what I said in my first post.
Buprenorphine definately is used in pain, and (like I said above) it is offten in such instances used in much lower dosages than when it's used for replacement therapy. The reasons for this are mostly due to the fact that bupe has a cieling effect, and at lower dosages, their is more of a spike each time it is taken than when the daily dose is above the point where analgesic effects plateau (due to the fact that once you're significantly above this point, you feel no withdrawal symptoms until enough time has passes and enough of the drug has been eliminated that it dips below this cieling. Basically, at these dosages, the receptors are saturated, creating a sense of normality for people who took opiates recreationally. This is a problem for pain patients though, because no only will taking more buprenorphine be ineffective, but it will also block the effects of other opiates.)
