After sex I usually have weak urine stream for about a day. Is it a prostate symptom?

Interesting. Could be due to a syringocele, which is a urthral swelling from a partially blocked cowper's gland duct. Other possibilities are blockage of the ejaculatory duct or even temporary failure of bladder neck to relax. Bladder neck remains closed during ejaculation to all semen to flow out of the urthra rather than backwards into the bladder. See a urologist symptom increases in severity.
Might be. This could be a mechanical phenomenon from seminal fluid in the urethra, although this usually would not last for an entire day. Alternatively can be due to inflammation in the prostate, aka prostatitis. May be spasms of smooth muscle of prostate and pelvic floor triggered by ejaculation--in order for strong stream need coordinated bladder contraction and prostate muscle/pelvic floor relaxation.