Ron,
The likely answer is two things may be happening. First, if your shower has a temperature pressure balancing valve, the toilet flush takes a large fast flow of cold water, and that causes a temporary drop in cold water pressure coming into the valve. To prevent scalding the valve balances the HW flow to correspond to the ratio between cold and hot before the flush. That would be normal.
But if you have noticed this change gradually over time it may also relate to a water logged accumulator or expansion tank. They not only protect the system against excessive pressure as water heats in the HW tank, but they help level out the water flow to a constant. I don’t know if your water pump has an internal accumulator or if your converter installed an external one, but it might be worth seeing if that is contributing to the issue.
Over time accumulator tanks need the air pressure adjusted. With zero water system pressure and an open faucet the proceedure is to inflate the accumulator tank bladder to the pump cut-in pressure.