Feeling Odd After Penetration

Ellora
Whenever my boyfriend fingers me and goes in as deep as he can, I have to tell him to stop because I just get this odd feeling like something is physically not right in my stomach (physical feeling, not emotional), and that feeling doesn't go away for a while. It happened when I used to finger mysef and I just stopped doing it because of that discomfort. 
I've never had sex, so I don't know if that's something that goes away after I do, or if that's a problem with my body. 
Any ideas/knowledge?