It will depend on two things. One is what time in your girlfriend's cycle she was -- if she had not ovulated that day or did not ovulate within several days of the event, you would not have gotten her pregnant. The second is whether there was sperm in your pre-ejaculatory fluid. If you had not ejaculated within an hour or two before this and had peed several times since your last ejaculation, your pre-ejaculate should not have sperm in it. Read up on Cowper's Gland and the fluid it secretes so you know what I'm referring to. It's good to be cautious, however. Putting your fingers in your girlfriend's vagina whether or not you have just touched something questionable is not a great idea, it can also give her a nice little bacterial infection. Fingers just touch too many things to be clean.
very minimal chance. not likely at all but it does rarely happen.