Hi NoisySmurf and welcome to JP I hope you enjoy it here. I'm Mandy, mum to Lauren almost 3 and Jake 11mths.
In answer to your question. The average ovulation day is day 14 when your cycle is the average of 28days. Therefor if you have a later than average ovulation day but an average length cycle it is possible it is still to early to pick up enough hormone on the HPT which is why you may be getting a
bfn. I would leave it another couple of days and if still no
AF then test again hun.
As for the constant nausea it could be a pregnancy symptom although tbh nausea isn't always the first symptom to rear it's head. It is also perfectly possible that you have picked up a bug. There are lots of sicky bugs going around at the moment.
Also stress can affect your cycle and even stop you coming on. If you are really stressing over getting pregnant then this could be having an affect? Which is why you may not have had your
AF yet. I know it is easy for me to say but relax and enjoy the sex

and fingers crossed you will fall pregnant shortly. Good luck hun, sorry i couldn't be more help!

Xx