Hello and welcome to MSF!

It’s very understandable for you to look back on this, try to make sense of it and grieve it.

I got a sense from what you wrote, that your friend felt jealousy about your new relationship. Maybe they felt sad for themselves having to cope with their caring for their parent situation, or grief. Then when you were happy, they felt envious. Plus it was not only a platonic friendship, there was some romantic history.