Sometimes physical tension is influenced by mental tension. So, find a way to relax yourself - be it relaxation techniques or music or swimming or which ever that suits you best. Yoga and meditation can be helpful too. If possible, find a few minutes during the day when you relax yourself and this will contribute tremendously towards easing tension.

GOT A COLD
It is possible to sing when you have a cold although you may just not feel up to it. As long as you don't have a sore throat as well, you can sing. In fact, with the right sort of training, many professionals find that they are able to have the technique to sing with a cold and can do so without too much difficulty. In fact some singers feel their voice sounds better with a cold as the production of mucus affects resonance. 

If you are going to be singing, don't stress yourself out. Sing what is comfortable and at a range that is comfortable. Keep drinking fluids so that mucus doesn't turn 'sticky' because it can be a bit of a nightmare trying to get of it!!
However, on the other hand, if you just don't feel up to it, listen to your body and don't sing!

SORE THROAT
If you have a sore throat, this could indicate a throat infection in which case you may need to see your doctor and rest your voice. It is very important that you do not sing with a sore throat. If you have persisting sore throats, this may be a sign that you are misusing your voice, or that you have been experiencing some sort of stress, or that you have a medical problem which needs attending to. In this case, you should see the doctor.