it can be effected by several things. your distance from a water tower, amount of water in the tower, the size of your water lines under your house. and last build up in your lines slowing it down. but if you only talking about the water pressure dropping when you use it from multiple places at the same time that is normal. as far as it getting cold, check your water heater, there's 2 elements in most of them. if one is working you will have some hot water, but not enough to have a shower and run a washer.
i left out one more possibility, if your bill is really getting high you could have a leak under your house.