TMI - bleeding after sex

I'm 21w pregnant today and last night me and hubby had sex. All was fine at the time but when I got up to use the toilet in the early hours of the morning I found watery blood when I wiped. Since then I've only had a couple of bits of spotting and then this evening I found really brown discharge when I went to the toilet. I know brown blood is usually nothing to worry about as it's old blood.  I've also been experiencing cramping in my lower abdomen all day as well. Baby is moving a kicking fine. What do you think, should I be worried? Thanks