Hematoma. Can I use a hot water bottle for the pain or is a cold pack better?

In my opinion, if you have a hematoma it needs to be immediately drained. If not drained, this can lead to infection. You should focus on getting back to the plastic surgeon.  He if she should attend to this hematoma. This will improve your pain.

If you indeed have a hematoma this should be surgically drained. Any sort of application would be for comfort and is not therapeutic.

If you have a large hematoma then it should be drained surgical.   If you just have bruising then using a warm compress may be beneficial.