I've had light spotting off and on the entire pregnancy. Call the doctor if it doesn't stop or if it gets worse. Otherwise, my dr said it isn't a problem.
I'm 33 weeks and had one literal bright red blood spot yesterday. I called the hospital they said if not paired with back pain contractions or worsening bleeding just keep an eye on it and try and stay off your feet. Don't stress like I did and have a melt down. I'm sure you're fine and there's many causes for it. Sex or pelvic exam or just an irritated cervix.
My OB mentioned that as your cervix starts to thin out and open, you could experience some slight spotting. This is probably more common closer to 36 weeks though. Sex or any type of cervical irritation (vaginal exam) could also cause spotting. I would call the doc just to make them aware and ask if they'd like to see you, just to be on the safe side and for peace of mind.
I agree with pp that it's probably best to at least call and let your Dr. know so that he/she is aware of it. They may want to see you and they may not. It's probably not at all a cause for concern, though, so try not to stress yourself out about it.
Re: Light spotting at 31weeks?
It's probably not at all a cause for concern, though, so try not to stress yourself out about it.