Usually stomach pain after sex is common during first time intercourse or sex after long term abstienence
This kind of pain if occurs suddenly after having routine sex should be investigated.
This kind by due to some infections or since you said something is lower than usual this can be descent of uterus
or change in uterus position, this may be due to age or repeated pregnancies in past.
Since you have negative HPT it is unlikely that you are pregnant.
If the pain persists you may need a GYnc examination and Ultrasound