OP, where are you located. I've been across the country, and there are some areas where the heroin is just shit period. In my experience, the more rural the area, the less pure and available the heroin is. The heroin that you've done while on Meth was probably of low quality. Even with the intensity of a meth high in the foreground, the impact of good heroin would be noticeable.
I have found the effects of combining heroin and meth, or opiates and amphetamines in general have been interesting. The amphetamine high seems to always be in control, unless I do a much larger than normal amount of heroin. Then, I can be knocked out. The advantage of the opiate is that it tends to mitigate a lot of the negative effects of the amphetamines. Teeth Grinding, Nervousness, Motor-Mouth, Racing Thoughts and the inability to sit still seem to lessen when thrown into the mix.