Spotting after sex/masturbating?!

I feel odd putting this out there. Lol. Anyway, I'll be 28 wks pregnant monday. I haven't had sex much lately as I feel huge. Sometimes I just use my vibrator. Totally outside stimulation only. I did it last night and a few spots of blood was on the toilet paper when I wiped. I've had 4 kids and this has never happened before. Doesn't make sense seeing as it was only outside stimulation. Not sure whether to think the blood came from the inside or maybe it was too rough on some part of the vagina and that's where the blood came from. It freaked me out pretty badly but wanted to know has this happened to anyone else and is it a real worry?  Just had remnant of blood when wiping the next few times. Everything seems ok now but I'll be scared to do that again, or even have sex (hubby can't wait forever). help!