Brown discharge at 31 weeks

Hello everyone,
Tomorrow I will be entering my 31st week of pregnancy, so today I am 30+6. This morning I noticed some brownish discharge on my underwear, without any clots, blood, or anything similar. It was as if there was just a small brown stain. Since it is brown in colour, I assume it may be old blood, but I haven’t had any coloured discharge at all during this pregnancy so far (not even in early pregnancy), so I am worried.
The baby is moving normally and I do not have any pain. The only thing I am experiencing is a burning sensation when urinating, but that is due to a urinary tract infection that was diagnosed at my last appointment.
I saw my gynecologist this Wednesday and everything was fine, but a transvaginal ultrasound was performed, so perhaps the examination caused a small amount of bleeding that remained afterwards.
This is my first pregnancy and I don’t know what to think. Do I have reason to be concerned, and has anyone had a similar experience?
Thank you in advance.