I am 18 weeks pregnant, while I was trying to look underneeth the bed I accidentally laid flat on my stomach on the floor for a few seconds. Now I am freaking out since I have mild pain in my right lower stomach and back.
Should I go to the ER? It was like 7 hours ago..

You should be able to lie on the floor without harm, you will get plenty of aches and pains, and you should be in regular communication with your OB/GYN, not the ER (unless much more serious symptoms). The ER is just not going to give you what you want and need for this, the staff will feel frustrated, and the costs are astronomical.
Leave a message with your doctor, take a warm bath, and see how you feel the rest of today.

Is there something in your history such as miscarriages, or anything else?