What Causes Frequent Urination With Blood In The Urine?

Im 37 yro male and yesterday i had to urinate very bad and was pinching my penis to prevent any urine from coming out. At this same time I was bending down to sit on the toilet and felt a sharp pain in my penis. I then urinated and when I stood up I noticed a small drop of blood on my penis and some in the toilet. The rest of the night I only urinated while standing and did not have any blood in my urin or on my penis. All today while urinating while standing I had no issues with blood but tonight I sat on the toilet and once again I had blood in my urin and on my penis and then about 2 hours later I urinated while standing and there was no blood so Im wondering what is the possible issue did I damage something when I was pinching my penis or is it more likely that I have an STD.