Hello there

There can be a number of different reasons for irregular or bleeding after sex. Things such as hormonal changes, infections, vaginal dryness, ectropians and so on that you can read more about here and here. 

It is reassuring you are up to date with your cervical screening so it is unlikely anything serious is going on so try not to over think things for now. At the same time I would suggest you arrange an appointment with your GP if this is continuing so they can check you out and arrange more tests if needed.

All the best

Naomi