@Genie1 Are you using Levemir insulin ?
If you are - do please refer to the leaflet in the box packaging .
It will explain about the preservatives in this insulin
can cause
a red swelling site area .
This is listed as a patient allergic reaction to the preservatives in the
Levemir insulin .
See side effects listed ...
I use Levemir insulin and get this happening to myself ,
just exactly as you described in your OP .
Red and warm areas , these can itch and feel tender .
They can swell up larger over 72hours before fade out .
Always inject your insulin 'slowly' count off up to ten before remove
the needle too .
This will prevent any stinging , bleeding at site puncture and the count of 10
will ensure all insulin is administered into your subtaneous fat layer securely .