Worried about My Inflating Belly

So I've been doing inflation for about a year now, filling myself with air and distending my belly, but something happened that worries me.

Normally, when I inflate, my belly begins to ache when I go too long. But last week, I was inflating for about 20 minutes. At that point, my stomach begins having its usual pains, but this time, it didn't. Not like I'd adjusted to it, but I inflated for about 35-40 minutes before I knew it was time to stop. I had myself checked out, but nothing ruptured.

Is this normal for veteran inflatees?