Think about replacing it! My guess is that either you put a new seat on it, or someone applied some sideways pressure to the existing seat. With a new seat, you may have tightened the bolts too tight or didn't install the resilient gaskets that usually come with it. Is the seat a heavy painted wood one thatgot slammed down one too many times? Are the rubber bumpers still there? Both of them? Significant twisting of the seat can occur if one or more of the set of rubber bumpers on the seat are gone.
If it was cracking around the flange bolts at the bottom, it could be that something eithermoved the drain line, or there may be some rot and the floor around the toilet is sagging slightly, but you indicated it was up top. If it is a two piece and it is cracking from the back towards the area where the seat is bolted down, someone may have really banged against the tank (maybe a really big person collapsing onto the toilet or really slamming into the back).
New toilets will use probably less than 1/3 of the water your old one is using. If you pick one with good specs, it will work as well, too (there are some dogs out there).