When I ejaculate it feels like I'm urinating. The "orgasam" feeling is usually dull over gone

I'm 16, 4 months ago I started to have a pain in both of my Lower sides, I noticed I was urinating a lot more frequeany and I felt the urge to go more often. Around the same time I noticed when I Masterbate I feel like I have to urinate and when I ejaculate it feels as if I'm peeing, and the feel good feeling is usually very dull or gone completely. I went to the doctor but I only told him about the pain in my aides and he thought that I had a UTI and gave me an antibiotic shot. The pain in my side went away for a week and came Back, this time I went back to the doctor and he had me Sent in for an ultrasound on my abdomen & kidneys. They didn't find anything, a moth went by with nothing changing and then I began to have a discomfort in my Lower stomach, almost as if I have stomach gas, the discomfort usually gets worse at school when I'm sitting at a desk for A long period Of time. When I lay down flat The discomfort goes away, I went to a urologyst and told him about the pain in my sides and the discomfort in my Stomach, too embarrassed to say anything about the ejaculation problem, and he didn't find anything wrong. Sometimes it takes me a while to start urinateing also. Here I am 4 months later with the same problem. My family doctor suggested an MRI. Could it be prostititus? Any Advice or suggestions would be greatly appreciated. Thank You.