Low back pain

A couple of weeks ago we did heavy deadlifts at Crossfit. And one thing I can do is heavy deadlifts. So I pushed it. The next day my low back hurt. Duh!!!! It went away fairly quickly though, so I was all good with that. The following week we did deadlifts in a WOD and I went heavy again. And guess what? That's right, my back hurt the next day. Duh!!! Again, it cleared up fairly quickly so no worries. But ever since then, when I get up in the morning my low back hurts. Actually, it's more stiff and inflexible. After getting up and moving, it's fine and then I forget about it. Until the next morning. I'm wondering if our mattress is done. We've only had it about 4-5 years, but I'm thinking it might be time to consider a new one.