Running 'pace' can be affected by so many different things.
Viral infections can play havoc on your fitness. Often it is in your recovery phase, where placing additional 'loads' on the body system by over exercising allows the virus to take get a hold again and it slows down your whole system.
If you over run and don't rest in-between sessions your legs will feel like lead weights
Diet is critical too - lots of fresh fruit & veg and plenty of water
If you have put any weight on it will slow you down
If you have changed trainers and they are a bad fit or excessively cushioning it can slow your pace
If you run on different terrain, awkward junctions, obstructions, slippery surfaces it can slow your pace
Be patient, alter your pace, change your length of run and above all keep enjoying your running. Make running pain free, and free from injury your goal. If you have had knee problems get a knee MOT from a good physiotherapist. Run to enjoy and ignore the timings.