The cause of water pressure suddenly increasing in a shower is usually the restricting valve being stuck or broken.
New showers that are installed and new homes have restricting valves installed which help control the water pressure in the shower.
Sometimes a restricting valve can be repaired while other times the restricting valve may need replaced and overtime these restricting valves wear out.
Sometimes it may be a problem with your low flow shower head as well but usually it's because of the restricting valve.
Low water pressure in your shower can be caused by a clogged shower head, partially clogged shower head, a restricting valve or someone using water in your house at the same time you're taking a shower.
If you're taking a shower and someone turns a faucet on in the house or flushes the toilet or turns the washing machine on then your water pressure can temporarily drop until the water faucets turn back off or until the toilet finishes filling up.
If you constantly have low water pressure in your shower it's likely that your shower head is clogging up with calcium from hard water.
You can soak your shower head in some CLR or vinegar to clean the calcium deposits out of the shower head.
Take a bag such as a Ziploc bag and fill it halfway with some CLR or Vinegar and then tie the bag around the shower head.
Allow the shower head to soak in the vinegar or CLR for about 8 hours or overnight and then turn the shower on with the hot water and allow it to push the calcium deposits out of the shower head.
Sometimes you may need to put a new shower head on if you already have low water pressure and a lower flow shower head will increase your shower water pressure when you have low water pressure.
Normal shower and home water pressure should be between 45 PSI to 55 PSI.