Spotting in third trimester

The other day during my midnight bathroom trip I kind of saw some faint red color when I wiped. As I was in deep sleep, I didn't quite understand what it was and went back to sleep. Next day morning I realised it wasn't a dream but it was in fact light red color. But after that one time I never had any spotting or bleeding. So I waited all day for it to happen again so that I can call doctor. But nothing. It was just that one time. Any ideas on what it could have been