I'm on 20 mg IR generic adderall twice a day. Lately, I've been eating a light breakfast around 6:15 and taking my first dose around 7:30. I take my second dose at about noon, usually skipping lunch due to lack of an appetite. I like to exercise at night, so around 4 pm I've been forcing myself to eat something small (maybe 200 calories) like oatmeal, so I have energy to run when I get home at 6 pm. However, I have noticed that when I do this I usually get pretty bad heartburn/nausea about 15 minutes after I eat...

Has anyone else experienced this or know what might be the issue?

I don't know about the heartburn, but I'd guess the nausea is due to not eating between 6:15 am and 4:00 pm. I get nauseous when I go too long without eating. Why don't you eat lunch before you take your second dose?

^ I usually just have no appetite whatsoever by lunch... and the weird thing is I get nauseous after I eat... if I don't eat, I don't feel sick, but then I have no energy..

Try eating healthy snacks throughout the day. I know the addy eliminates appetite but try planning times to eat a couple handfuls of peanuts or yogurt/cottage cheese...something w/ protein that won't digest in the blink of an eye. Just because you don't have an appetite doesn't mean it's physically impossible to eat something anyway. Drink as much water as you can.

Eating that little on stimulants can be counterproductive. Try pre-packaged protein drinks like Ensure if food makes you nauseated.

You can't deprive your body of food/nutrients and expect to get benefit from exercise as well. Odds are, the nausea will subside if you start eating at more regular intervals.