Brief Answer:
It can be a local infection.

Detailed Answer:
Hi, First of all thank you for approaching us for your query. I read through your query. I suggest you not to get tensed about it. First of all there will be difference in the fitting and seating of new crowns(how much ever perfect dentist does there bound to be slight difference from the old one as it would have got accustomed with the oral tissues).The old crowns would have got adjusted to your gums and the gums naturally would have grown around the crowns. When we put new crowns there will be difference in fitting and it will impinge slightly on the gums which will irritate your gums. Moreover this irritation will cause local wound and aberation on the gums which can lead to slight local infection. I suggest you to do warm salt water gargle for a week and take amoxycillin for 3 days. Apply metrogyl gel on that swollen gum area twice daily for a week. Also please dont use that side much for biting for a week till it heals.It will be fine within a week or so. If after following still its not getting fine then you can meet your dentist and get it treated. Be calm and stay happy. This is just a normal scenario which happens when we replace long standing crowns with new crowns. Happy and cool mind is best solution for any problem. Hope you understood what I meant. Have a healthy day. Thank you

---