Hip/back issue- is it just a posture issue?

Im 19(F), 5'4, not sure my exact weight but id assume average-very slightly overweight? Ive had no previous medical issues and ive never taken any medication or tablets(inc painkillers).
Added info: my work requires me to sit for multiple hours every day, i rarely exercise, and in my freetime i play games which i sit down for hours at a time too.

For the last 2 months ive had REALLY uncomfortable back and hip pain whenever i sit down for long periods of time and it becomes worse when i slouch.

That in itself is bothering me a lot but the worst part about it is when i get home from work and i lay in bed for maybe ~5mins my hips hurt expodentially more and when i stand after lying down i get a continuous stabbing pain in my thighs and hips that make it so painful to stand and walk. After 10mins of standing/walking the pain goes away until i sit/lay down again

Theres also been a massive increase in big pink/purple stretch marks appearing on my legs; not sure if it has anything to do with my issue so thought id add it(i would add a picture but im at work right now).

My question is: is this a sign i should work on my posture/exercise amount or is there maybe a more serious problem occuring?

(And sorry im super bad at explaining stuff)