“Why do I get stomach pains every time I sleep over at my boyfriend's house?”

Every time I go over and spend the night with my bf (I’m not sexually active) I always (every time) get this painful stomach ache that’s almost like cramping or like a sharp pain in the middle-upper area of my stomach. The first time it happened, I got it that night. But every time after that (like 5 more times) I noticed it the next mid-day. I’ve never gotten this type of stomach ache in other instances like nerves or anything. I’ve never felt the need to use the bathroom or anything but I’ve felt a bit nauseous before and I had trouble eating. I have spent a week with him and I did not get this stomach ache, that was the only time I didn’t get it. Every time I spend one single night over, it happens without a doubt. I eat dinner with him but it’s different every time. I have absolutely no idea why I get this but it makes me frustrated and it’s horrible.