My girlfriend and I had protected sex the day after her period stopped. I came in the condom, took it off and cleaned up. This was all fine, however the next morning, we went again but it was unprotected - however i did not ejaculate inside her. My worry is that there could have been leftover sperm on my penis from the previous night - is this possible?
Yes, it is possible. Did you urinate after ejaculating earlier? It can help flush the urethra. There is always a chance however, with any sexual contact.
The timing, for her to conceive, wouldn't likely be that good. Assuming a 6 day period, you had sex on day 7 of her cycle, she'd likely ovulate between days 11 and 16. Sperm can survive about 5 days so if she ovulates early there is a chance.
I don't think the chances are high but it is always possible.
Hope it helps.