Well done Aimee, so glad it's going so well for you. I would hate for you to think I was discouraging you in any way, but take it easy for a while, whilst you get back into it. I'm sure you'll be up to 5K and more very soon.
Keep running with your run group. Are they able to offer any advice to you? I wouldn't run much further than about 4 miles until you can do the distance comfortably. Put emphasis on running regularly, perhaps 3 times per week would be good if you can manage it.
On one of your runs, (assuming 3 miles) run the middle mile a bit quicker than normal, with a nice slow warm up and a good warm down. A bit of speed, relatively speaking, will add another gear to your performance.
My guess is that your lungs are tightening up, possibly because of the cold or being unwell and that you aren't noticing it until after your run. It's a bit like a pain going away when you do a physical activity, the adrenalin masks the problem.