I figured that I needed engine oil change and took my Toyota corolla 99, to the mechanic.
The shop offers free brake check, engine oil change and some minor free service for 30$.
After picking the car,I felt the pressure on the brake was less compared to how I brought it, but drove to work with it,I try to apply pressure while driving to try to bring it back to normal.
but while returning from work and almost home, the brake light showed up.
don't want to panic, can it go off after some days or it needs diagnosis.
what can I do manually about it myself

I advise you check your hydraulic fluid level and also be mindful of your brakepads..

@op, u already said it.......brake oil level......shikena! Just add oil to max level, datzall!