You're probably under stress for some reason. Processing noise takes brain power, and if your brain's being forced to work on something else, or if it's not doing as well as it normally would be, then you have less energy left over for dealing with noise.
Physical stress and emotional stress can both cause this. You're coming down with a cold, or it's been uncomfortably hot, or you're not eating an adequate amount and quality of food--that's physical stress; you can't deal with as much as normal. If you're female, your period will do it too. Cognitive stress--having to deal with a lot of changes; having had to spend a lot of effort to learn something new.
Whatever's going on, try taking a break. Spend some time alone, doing something low-key. Get a good night's sleep and eat three square meals a day. If sunshine and fresh air relax you, go outside; if exercise relaxes you, get some physical activity (but not too strenuous, because physical exhaustion would cause overload too--try going for a walk).
I think this happens to everybody, NT and autistic; it's just that autistics have to work at nearly full capacity just to cope with everyday life, and when things get stressful, it's much more obvious.