Mid to upper spine pain, for 9 weeks so far, pain when bend forward. sudden, no injury. could it be due to posture or others?have slouch posture. 25f

Possibly yes : That would most likely be responsible for it, in absence of an injury; unless you have started work involving a lot of slouching over, as in computer use or reading etc. Since you always slouched, its now causing pain. Help yourself by not doing so and discipline yourself to avoid it.Therapy would be helpful. Back strengthening exercises & swimming (backstrokes) + nsaids will certainly help.

Yes: In the absence of trauma, accident, lifting/exercise injury- poor posture and daily spine hygiene is a common cause of precipitating muscle spasms and pain in the upper back. Good idea to try getting up and stretching regularly thru the day, have an ergonomic eval and implement an ergonomic work station. Don't sit in one position for >30 min w/o changing positions or stretching. Go see your doc.