Ummmm...nopes. I don't think you're over reacting at all. It is NEVER ok to be threatening or violent towards one's partner. If it's your belief that the drugs cause the behaviour in him, I say you're well within yours rights to request him to stop, however rare his use is. The fact that he then got violent and threw the phone at you - I'd say there's an issue to deal with as well as the drugs. You're not his punching bag, nor should you feel like you have to be because you're pregnant.
My previous DP and I fought a lot because he brought someone into our home who then brought a lot of weed into our home. She would get him to smoke it with her on our balcony, both of them knowing full well how I feel about drug use of any kind in my home. I was disgusted and said I was going home to my parents place for a few days, and when I returned either she was gone or I was. You have a right to feel safe & comfortable in your own home - I demanded it, and so should you!
You're gonna be a great Mummy. xxxx