First off, congratulations to you and your girlfriend for the little blessing. Well, that also gives another cause to rejoice. If you got her pregnant that means your reproductive system is okay. Though, most of your fear must be from having had unprotected sex with a random girl during the cruise you went to with your girlfriend. You know the risks so, your mind might just be bothering you about it and making connections with your current situation.
But if you want to be sure, you could get a DIY test kit to check on your prostate health. Try the Prostate Specific Antigen Test (PSA)-Home Blood Testing Kit of ZRT Lab for that. It's something you can do yourself at your convenience and for your own peace of mind. You should also consider that it might not have anything to do with your prostate health, and might be caused by something else, say muscle related or even a damaged nerve.