Shivv

I just bought a house, and the pool/spa has a Pentair FNS Plus DE filter. My pool guy just replaced the relief valve on the top of the filter (the previous owners had put the pressure gauge directly into the filter without a relief valve). I noticed today that when I turned the pump on I can visibly see the filter expanding when the pump turns on and contract when it's turned off. Am I correct in assuming that what is happening is a bad thing? I'm assuming that the new relief valve is holding pressure in the filter better than just the pressure gauge, which is causing the expansion. I just worry that the filter will eventually crack and fail and would like to get ahead of it. The pressure is right around 30 psi, which means my pool guy better backwash it when he's here again on Monday.