Pregnancy is a finicky thing, it could be her first time, her one hundredth or her five hundredth time when it happens. For me I've been sexually active for 5 years, used little to no protection and didn't get pregnant until I intentionally tried for 8-12 months.
Go to a fertility specialist. Be prepared to jerk off into a jar. (If they are close by, you can do that at home and bring a jar in). They will give you a sperm count and a motility level-which indicates their health.
sounds like you are but the only way to know for surfe is to go to the doctor... maybe donating at a sperm bank would give you the required info at a cheaper price because I'm sure they do some kind of test to make sure your not shooting blanks