What Causes Blood In Semen?

I m 48 years old and race mountain bikes. I haven t rode in several months but started back a few weeks ago riding on the weekends to get ready for an upcoming race in March. I ve noticed the last three or four times there s been blood in my semen after sex with my wife. I ve had this same experience a few times before but this the longest it s lasted. My testicles are not painful, there s no blood in my urine, my groin is not tender or sore and there s been no fever or anything of that nature. I wear biker shorts and protective equipment but the terrain is very technical and I know my testicles and groin are under constant heavy pressure with my seat. If I continue to see blood in my semen how much longer until I should make an appointment with my doctor?