5 month old pitbull puppy may be ill?

I have a 5 month old male pit who is generally very happy and healthy. Yesterday evening he started gagging and threw up a small foamy white mess, twice. After that he seemed a little sleepy but didn't have any more issues. This morning, he ate breakfast and was ill again, only this time he threw up his food as if it weren't even chewed at all. he has done this before because he eats too fast. I changed his food last week because the store didn't carry his usual brand, then a few days ago I switched him back to his normal stuff. He has had all his shots and has never been sick before, I accidentally skipped one month of his heartworm pill but gave him the next dose 10 days ago. Hopefully he just has an upset tummy but I'm looking for any advice, I hate seeing my baby sick!