A gap between my dental crowns has started to open and cause occasional bleeding, what do I do?

there are many reasons for that , so check-up is must to know exact reason. but shifting teeth is natural process of body [like wrinkles on skin], so with age spacing in teeth is common. you can close these minor spaces by aligner treatment. remember dental is not a treatment, its investment. if you want good oral health you have to invest in that time to time. regards

If you have spaces between many of your natural teeth, then these teeth can move around and cause spaces to develop, regardless whether you close one space with a crown. To properly close all the spaces, you either have to have some type of orthodontic treatment or Invisalign.  Once the spaces are closed and narrowed to one space between two teeth, then you can properly crown the tooth/teeth to close the one space between them.    
If you have only one space and the spaces between the other teeth are all tight, then maybe you can have the one crown done over to properly close the one space.