My kitty has just been through this.....the yelping bit lasted 2-3 days then she calmed down. I found a heat pad in her basket helped, as did a good sprinkling of cat nip leaves for her to roll around in.
Mine was booked in to be spayed today but vet has discovered a heart problem so it on hold whilst she has more investigations.
Cats are induced ovulators (yes it takes sex) and can get stuck in heat until they ovulate so you could be stuck with this. Cats protection have lots of campaigns on at the moment we are running one for them where she would be done for a £5 so give them a ring and see if you are eligible.
Had a bit of a panic last night because of a total change in behaviour with Kitty (new cat owner alert!) so I had a quick look online and I think she may be in heat (body low, bum in air, tail up, yelping and generally looking sorry for herself). She's 5 months old (ish - we're not sure exactly) and due to be spayed in 2 weeks time. She hasn't been outside yet and won't be till she's done, she's eating and drinking like normal but the yelping is driving me a little crazy. Is there anything I can do to relieve symptoms or distract her from yelping all the time so both me and the cat can get some sleep tonight?