[URGENT] condom slipping

Okay so me and my boyfriend had seggs and we changed positions. When i had put it inside from the back the condom was surely there. We did it for like 5-10 minutes and he pulled out slowly and we rested for some seconds but the condom wasn't on his penis when he looked down. He said he didn't feel anything changed while inside of me and neither did I. We quickly got up and i noticed the condom was hanging halfway outside of me with the ring part. I'm thinking that maybe if the condom slipped during sex, it would have been pushed deeply inside of me. NEITHER of us have came. I am NOT on birth control. My period just ended 2 days ago and i have a regular cycle. Should i worry? Should i take plan B? I don't want to have sleepless nights because of this...