I had naked frottage 4 weeks ago with a female.

After 18 days I had std checks done via blood tests and urine samples which included chlamydia and ghonorea. I also had a 4th generation antibody and antigen HIV test even though you confirmed no risk. They all came back negative.

However today I masterbated twice and after each masterbation is squeezed my middle towards top of penis to draw out any remaining semen so that it didn't drop in pants which I have always done.

However after milking the penis the second time post ejaculation there was fresh blood came out of my penis. 

I have urinated several times since and there seems to be no blood in the urine plus there was no blood in the semen when I ejaculated but rather it seemed to come out as I milked the reminants of semen out after ejaculation.

I am now concerned I have chlamydia or gonorrhoea due to this incidence as I worry about accuracy of the urine test. Reason being that I filled up the tube with urine for the test and I have since read it should only be the initial burst. 

Could the test have been false negative due to amount of urine diluting the bacteria and therefore results false negative and this bleeding actually be due to the STI?

It happened earlier today and I am not bleeding any more that I can notice. There is no stinging or pain when I urinate either. 

Please help as very worried.

However, no STD causes urethral bleeding without also resulting in discharge; and as discussed in your other thread, there was no possibility of STD from the frottage event described there. Also, the standard urine gonorrhea and chlamydia tests are highly reliable. False negative results can occur but are uncommon; combining the negative result with the absence of risk, you can be confident you have neither of them, assuming no other (truly) risky sexual exposures in recent months. The details of urine specimen collection make no known difference in test performance or reliability. Anyway, in the extremely unlikely chance you actually have either of these STDs, it doesn't explain your transient urethral bleeding.

My guess is that you traumatized your urethra by vigorous "milking" after masturbation. If it doesn't recur, I would advise you to ignore it and not worry; and be more gentle after masturbation in the future. (Even better, don't squeeze the urethra at all. Urinating would make more sense to assure no dripping after ejaculation.)

Regards--  HHH, MD