I recently picked up a 5.7 M&P. I field stripped and inspected it as soon as I got it. Took it to the range and fired 50 rounds through it. I noticed the slide was getting hard to get out of battery when loading the first round. I cleared the gun and inspected it. I can barely get the slide to move out of battery now. The barrel rotates enough to unlock and then the slide stops. I managed to get it back after a few attempts but now my slidestop won't press out so i can field strip it to check it out. I tried a small punch and tapped it but it just won't budge. I pressed it out easily the first time I stripped the gun. What could be causing these issues?

Sounds like there is a lot of pressure on the slide. Maybe the recoil spring/rod is out of place?

The most obvious issue would seem to be in the gas port though I have no idea why 50 rounds would cause it to bind like that.
You stated you "managed to get it back..." which I'm guessing means you got the slide to retract.
First try spraying every nook and cranny with WD-40.
The tap the slide (if required) forward to align the little scallop in the left side with the little bump on the slide stop. Support the gun on a padded surface, grab a properly fitted square faced punch, and steel hammer (not mallet). Tap and dont be afraid to increase your blows - if needed. The only thing holding that slide stop in is a small wire spring - and very likely a great deal of pressure applied across the slide stop in by the barrel sleeve - which is locked in by the slide stop pin.

If you haven't already found this out by the time you read this, most likely when you start the slide forward, the bind on the slide stop pin will lessen or disappear.

If the slide has broken loose enough so that spring pressure wants to move it out of alignment with the slide stop notch, you'll want an assistant to stabilize the gun with the slide opened at the exact spot while you tap the pin in order to avoid strain, stress or damage to the slide, slide pin, or alloy sub frame.