Can ER be a good idea since I can't get to dentist until Monday

If there's an infection, you will need antibiotics to relieve that before any dental work can be done, so yes, go to the ER.

Once the infection is gone and you are feeling better, please go to the dentist!

In the ER I treat toothaches with antibiotics and pain relievers. So yes, go to the ER since you can't get into the dentist over the weekend.