A single episode of spotting/bleeding after sex is not worrying. Mostly it can simply be due to rough sex, due to injury during sex, specially if there is not enough lubrication. Also women sometimes have bleeding after sex if they have not had sexual intercourse for some time, possibly because of decreased lubrication.
However, if there is repeated bleeding, you should go for a checkup for this. It usually occurs due to chronic infections like cervicitis, due to thinning of the mucosa, cervical polyps, etc. It could also signify more serious causes like cervical dysplasia. Hence you need to get this evaluated as early as possible i you have repeated episodes, you should attend your gynaecologist as soon as possible.
For now, avoid rough sex. Use a lot of lubricants during sex. Preferably use condoms.
But do see your gynae for this if you have repeated bleeding, a gynae checkup is essential for this. You will require tests like a Pap smear. This is most likely due to infections like cervicitis for which you will need antibiotics.
Hope I have been able to clarify the situation for you and helped in resolving your query.
If you need more help, please let me know so l can continue to assist you.
Thank you and I wish you the best of health - stay safe!
Dr. P. Jyoti