Pressure was fine then all of a sudden it changed and was weak

peepm, you will need to give more information, like city water or well, type of piping, any plumbing work done recently? A lot of good help here but need info. Good luck

Yes, more info please. Any filters, or a water softener?

One bathtub (only the tub the sink pressure is fine) had low pressure and so I used the other tub. Now the 2nd tub has low pressure but the original tub with low pressure has more pressure now. The pipes are whatever they used when the house was built in 1971. No filters and no water softener. I have a tankless water heater. Thanks for responding.

One bathtub (only the tub the sink pressure is fine) had low pressure and so I used the other tub. Now the 2nd tub has low pressure but the original tub with low pressure has more pressure now. The pipes are whatever they used when the house was built in 1971. No filters and no water softener. I have a tankless water heater. Thanks for responding.

Sounds like some debri may have gotton into one of the tub valves, then cleared itself out of one of the valves. You may need to shut water off and dissasemble the valves (cartridges) and clean them and flush the valve body with valves or cartridges removed, then reassemble and see how it works.

I know how to remove the stem (I assume that is the cartridge). How do you flush the valve body?

I don't know if this is related, but when showering the water turns cold half way through. This started when the water pressure changed. I have no water running anywhere else in the house and this never happened before. Maybe I need to get the water heater checked. I have had it 3 years and not serviced since it was put in.

peepm, to flush, after removing cartridge quickly turn the water on an off to remove debris. Use rags or bucket to try limit the mess. Good luck