you can ovulate at any point in your menstraul cycle. Thats why having sex during your period can still end in pregnancy.
Wait a few days and test again, then test every few days until either your period arrives or a positive result. Thats all you can do.
Dont forget that stress (and trying to conceive can be stressfull) can affect cycles.
The tests do pick up on the hormone, but it vcan take up to 3 days for an egg to fertilise after sex, then it will implant and then your body reacts and starts to produce hormone. It could be that there isnt sufficient yet.