Hamstring pain on squats

[leckbass]

I havent been doing squats for a while due to a few lower back injuries. Yesterday i attempted some very light (135lbs) sets of squats and i had lots of pain in my hamstrings, i stretched before i worked out and did one warmup set with just the bar. It felt like they were going to tear right off. Any suggestions to help them loosen up?

Thanks