On avastin (bevacizumab) for ov cancer. could there be a spike in ca125 because tumors are shrinking/dying and spilling their content into blood stream?

Yes, it is possible: Ca 125 is a tumor marker which helps to make decisions about response or lack of it. But it takes 2-3 serial values to decide if the tumor is getting better or worse; one reading does not help as it can be rising both from tumor getting worse or getting smaller and releasing tumor chemicals, which is what ca 125 is. It takes 2-3 mos to be able to get a clear picture. Also watch other tumor aspects.