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.
For water pressure to drop due to an increase in water usage around the house is pretty much normal in my experience. But I did have a problem with a water line leak about 20 years ago. We had unusually high water usage in the winter months, and I called the Water Dept. about it. They suggested checking the toilets to see if the tank flapper valves had deteriorated and were leaking water down the drain when they shouldn't have, as well as inspecting the area on the property from the water shut-off valve to where the line entered the house.
Sure enough - I discovered we had BOTH of these problems. There was a "wet spot" on the lawn that I had attributed to poor drainage, but in fact it was where the water line had sprung a leak underground. Called a plumber and about $50 later that problem was fixed. As for the toilets, I found that by listening carefully, one can detect the sound of water draining when it shouldn't be. The sound is peculiar enough that once you hear it you can recognize it again easily. About $5.00 and a do-it-myself job fixed the issue.
Good comments and suggestion, gacpl.