Why my screen suddenly turned very weird?

Ok so, I cracked my phone months ago and got it fixed a few days ago . My phone is iPhone 4 but when I went to fix it, there was only iPhone 4s white screen avail. so I just took it . After the repair, it was very new but the next day while I was using snapchat, the screen turned into like a bright color effect. There was yellow, blue, red and so on .. I tried switching it off and turning it on again and it worked. I thought it was all okay when the screen suddenly turned black. I panicked a lot I tried switching it off and it did worked but this all contuinued until today. Today my phone got another problem. Lines started to form on my screen, it kept flickering and it keeps shutting down til I get very annoyed.

Please do help me !! I will appreciate all your answers a lot !!