Don't worry I used to get the exact same thing! It's anxiety, don't underestimate the power of anxiety! You're probably having anxiety or panic attacks when you're away from your boyfriend because of either uncertainty or just being in an important situation.
So! The first thing to do is acknowledge the anxiety, tell yourself you're not really 'sick' (even though you feel very very sick). Secondly, do something you wouldn't usually do if you had gastro or food poisoning, like go for a walk. Basically prove to your brain that you are NOT sick. Thirdly think about how you're feeling and tell yourself it's okay to feel like this, even try laughing at yourself about how 'ridiculous' the situation is.
Once you've shown your brain it's okay, you're not really physically ill and it's funny and not a big deal, it will start to go away. If you struggle with it still I HIGHLY recommend seeing a doctor about managing anxiety. Please trust me!!!
Best of luck!