The last few weeks our water pressure has acted weird. The only thing I can think of is when you first turn on the shower it's great. Works awesome. Wait the few minutes to heat up and it's still great. Then it goes way down. It feels like as soon as the water heater gets down enough for water needed to start going on and filling it up the pressure goes down. But that shouldn't be the case. The feed line for the Water heater shouldn't effect the water pressure right.

Any ideas what else could be causing it?