Sometimes when I make love, my hips go out of place and I get a strain in my hip muscle.
Holding yourself in the same position awkwardly for a long time can put strain on your lower abdominal muscles. It could even be the way that you move while making love; if you use your hips and not your back, then you will probably stress your hips too much.
I know that if I am in one awkward position too long, under my shoulder hurts, deep inside.
Do you think that both of you could just be straining yourselves?
I think it is kind of odd, though, that you both have had bacterial infections with it. Maybe the infection just made your cervix (or other muscular structures) sensitive and sore, and it feels like your lower abdomen is to blame?
I am sorry I can't really give you a definitive answer. I just think it could be strain-related or possibly pain and soreness associated with the infection.