Do you ejaculate regularly during sex then if you're not doing so through masturbation?

As I mentioned, I stopped masturbating 3 months back so that rules out as an option for ejaculation. About sex, well i'm still a virgin.

Any idea why i'm having this problem?

My guess is that since you're not ejaculating on your own, your body is doing it automatically, likely in the form of a wet dream. Because nothing "forces" out the semen during a wet dream (it simply oozes out), you likely have extra semen left in urethra. Since the urethra is for both semen and urine flow, the urine likely forces out extra semen that is left in the urethra. Now that arises two points:
1. If you just masurbate once ever week or two, the problem will liekly go away.
2. Are you sure it's semen, and not a milky white urine or other substance? This might be a sign of an infection if it's not semen.

No i'm not sure if its 100% semen. Any way I can check?

I don't think its milky white urince because it is clearly different in texture from the regular urine and floats in the water like soap foam.

Also, why have I not experienced wet dream as yet as its been quite long (3months) since I last masturbated?

Sounds like semen, but who knows....

You could have a wet dream, but like I said, the semen may not have been forced from your urethra, leaving it to be pushed out when you urinate...