Ok, so you have a male and female budgie in the same cage together, has your female budgie's cere the part at the top of her beak changed colour at all recently? Has it become a deeper brown colour and looking sort of raised and bumpy? If she has she is what we call in condition. This when they will mate and lay.
If she is ready to lay she will be looking for a nesting place, doe she come out of her cage and go to a certain place all the time, or try to hid under the feed dishes?