it better to ejaculate on panties rather than inside her vagina. if she took them off and wash them , and none of your sperm could have got up inside her vagina, you will be ok.
if you were just masturbating near her , thats ok, but if you were thrusting inside her, and pulled out just as your sperm ejaculated, then some sperm may have spurted inside her, in which case she could be pregnant.
A morning after pill, or pregnancy test will help to find out.

There is a chance she could get pregnant, yes. It isn't unheard of; "immaculate conceptions" can happen through underwear or even pants if the timing and the conditions are right!

Please be careful; if you aren't trying to get pregnant, you should always, ALWAYS wear a condom or use contraceptives.

If your girlfriend suspects she is pregnant, she should take a home pregnancy test about a week after her missed period. A doctor can do a blood test as well to confirm.