Hi, shirley,
mayhew - above, may very well have identified your problem. At this time of year , you cannot make vitamin D in your skin , it is your primary source. Sunlight in the UK , is too weak , between mid-Oct and mid- April. That means all winter , your levels are likely to be dropping.
Did you ask your GP what your Vitamin D level was ? If it was not tested , then I would ask for that test as a matter of urgency.
Vitamin D does have a marked effect upon your immune system, in fact vitamin D affects over 2000 genes in your cells. Every cell in your body , has a receptor for vitamin D , it is used to fight off invading bacteria, virus etc. and many other processes.
have a good read of this thread,
www.mumsnet.com/Talk/general_health/2421492-Vitimin-D-can-it-have-this-effect
It is a long thread , but you will see how many other people were affected by low vitamin D , how they were helped , etc. I posted quite a bit on there , it will give you lots of info. IF I were you , IF my vitamin D test came back low , then I would supplement with several thousand IU daily, in order to get my level up above 100.
best of luck.
BTBH
.