Loss of Water Flow from Showerhead

A couple of times lately I have been in the shower on the second floor, and the water pressure dropped to the point where the gizmo which diverts water from the tub faucet to the shower head dropped by itself. My pulling it back up again and holding it up resulted in no more than a trickle of water out of the showerhead and the faucet. I think this happened when my spouse flushed a toilet in the basement. Flushing in the basement also causes a dramatic reduction in flow at the second floor sink faucet.
Also, during the backwash part of the cycle on the water softener system, water availability is much reduced.
Our house is 25 years old, we have lived here for 15 years. We have a private well. The gauge on the expansion tank reads 40psi (with no water flow anywhere except maybe the reverse osmosis drinking water system.) These problems appeared at approximately the same time as the addition of the RO filter, but perhaps that is just coincidence.
I would love to understand what is going on.
Thanks, Teffy
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