Ok sunday I went to the ER for vomiting. The told me my hCG was10. Monday stared bleeding mild cramping went to ER to see if I was having miscarriage was told level was18.Is this breakthrough bleeding?

Needs follow up. If you have just missed your period, then this could be several things. A normal beta HCG will "double" (or get close) every 48 hours. You had a near double in 24 hours. However, with bleeding you will need follow up to become sure which of the following this is: 1. A normal pregnancy. 2. A miscarriage in the works. 3. An ectopic pregnancy. Follow up asap for additional labs!
It is very low. But rising appropriately, we look for at least 66% rise every 48 hours and u have met that criterion but get close follow up. Gd luc.