Weird running problem?

Just recently i started having this fullness in my stomach when i run which slows me down a lot, kinda hard to breathe as well. I feel it during the 1600m not during the 800m however. I've been running for years now and it's just recent and it's slowing me down anyone have any idea what it is? It's not because I've been eating before the run, I give myself 3 hours if i do eat before, and i don't load up on water either before it.