Why does my urethra burn after defecation?

For a long time, I have been experiencing burning in the urethra only during defecation. When I urinate standing up, there is no burning. I only experience burning in the urinary tract after pushing. It's like there's urine left inside and it starts to burn. Sometimes, after urinating, some more urine comes involuntarily and when that urine comes out, there is a very bad burning. Two problems come to my mind.

1. Prostate - But the prostate starts at age 25 and grows. I have 23 years old. If there was a problem in the prostate, burning would occur when urinating standing up.

2. Sexually transmitted diseases - Which is impossible. I've never had sex before.
Could this be the problem because I'm pushing too hard? I am waiting for your answers