Update made my phone freeze!

I have had no problems with my IPhone 5 but since updating it to the IOS 8.3 it keeps freezing and is just really sluggish.

I have taken it into the apple store who have said this is nothing to do with the update but that there is something wrong with my screen instead. If I press the home button and then open it back up it seems to work for about 5 minutes and then freeze again.

I find it strange that I had no problems until I updated and that Apple are now charging me £100 to "fix" the screen that, to me, doesn't seem like the issue.

Has anyone else experienced anything similar?

It also deleted all of my messages that were on my phone and I have been told that I cannot get them back either. I have used ICloud but this did not restore the messages that I needed.

Any help/information would be grateful!