You need to have an ultrasound of your pelvis to show what the inside of your uterus contains.
Considering you have antibiotics and are now experiencing worse pain, those antibiotics may be insufficient for what you need. You may have another problem entirely which is why you need the ultrasound
You need to go to the ER for this.
And that is the only way to get stronger (intravenous) antibiotics if needed.
an ultrasound is the first test you need done and depending on those results a CT may be ordered.
Also a more direct evaluation of the uterus might be needed (with a scope)
But there is nothing you can do at home for this and only delaying this will cause more problems.
There are many many causes.
A pregnancy which has aborted and is stuck inside the uterus, a fibroid or polyp within the uterus which has fallen off and bled possibly trapped in the uterus.
Or a severe infection by itself or complicated upon the other possibilities I mentioned
This does need relatively urgent attention in the ER.
This is not something to be treated at home
it needs a more urgent definitive evaluation with ultrasound
Please do not put off going to the ER
Best of luck. It is better to do it now than to have even more complications