Your change of time is fine on Tenofovir. The hotness in your stomach is probably from something you ate or you may just be thinking that the change is affecting it, so it's happening. I typically eat 20 minutes before I take mine so it gives it time to digest my food. I've take it as much as an hour afterwards or an hour before I eat and was fine. I'd be more concerned if you switched the times you took your meds if it was during a different meal (example: You switched from breakfast to dinner, etc.)