It is normal. The semen will change consistency with time. The latest research is suggesting this happens to release sperm in a phased approach - different components in the ejaculate turn to liquid at different times. Ultimately most of the semen will just flow out in what is called flow-back.

If this was more than you expected, it can be for various reasons; your partner could have ejaculated more than normal (abstinence for a while or highly aroused), his ejaculate could have been more watery that you are used to (this happens and are also related to heat on his genitals), you could be in an infertile phase with your cervix completely closed to sperm (not within the few days around your ovulation), the temperature in your vagina could have been unfavorable (alcohol use or general infection like a cold/flu), no upsuck by the uterus (you did not orgasm or orgasmed at the 'wrong' time) or you could have been highly aroused and had a lot of arousal fluid (there is a spot on the top end of the vagina that if it gets pressured, it will cause the vagina to lubricate a lot more so if you tried a different position or angle, it could have been pressured).

There is little proof that the amount or timing of flow-back have anything to do with your chance for getting pregnant.

You have a number of options: Lift your hips after ejaculation (pillow under your behind) or you can stick a menstrual cup up there to prevent the semen from flowing back.

Best of luck with ttc!