There’s a lot of factors that could be causing this:
Side Effects of Supplements, Gear, Food, or Medication – If your body doesn’t want something, it will get rid of it. Expelling through the urine is the easiest way for it to do that. It’s also the most frustrating and annoying. For example, if I take a vitamin D supplement at any dose, I end up urinating frequently. It’s likely the result of me having high calcium levels. High zinc does this to me as well. 50 mg tablets are too much for me. Certain medications, like painkillers and other NSAID’s have an effect on the renal system. Any negative experiences might not even occur right away. It could be 12-24 hours after taking the medication.
Stress & Anxiety – Mental hardships can often manifest themselves in a physical way. Frequent urination and hyperhidrosis are the most common.
Retrograde Ejaculation – This is where semen enters the bladder at orgasm. It is the result of a weak orgasm that can be caused by a weak erection. Semen in the bladder will irritate it and cause the sensation to urinate. The problem is that the thickness of it can make it difficult to filter out and it will take multiple times before it does.
Weak PC Muscle – Somewhat related to the above; if someone struggles with ED, uses the death grip, or engages in rougher sex, they are likely flexing their PC muscles too hard during sex. Especially if they’re trying to maintain an erection. If the pelvic floor is worn out it can cause incomplete emptying and the inability to hold larger amounts of urine.
Caffeine, Sodium, and Fluid Intake – As addressed by others in the thread. This will be individual. Some people can drink coffee all day and go twice, others would have to take out a mortgage at the urinal.
As others have suggested, consult a physician/urologist. Blood work, urine sample, prostate check, and bladder ultrasound. Edit: I missed the post where you've been to the doctor.