severe flu-like symptoms with profound malaise after intensive meditation

I have been practicing intensive shamatha meditation for many months now. On workdays I manage ~4 hrs of meditation, two very early in the morning and two at night. At weekends I do meditation for 6-8 hrs in intervals. I do have certain basic experiences like intermittent appearances of what I believe to be uggaha nimitta and very rarely what seems to be some early Piti which stays for some time and then vanishes. What I am troubled by, however, are the intense flu-like symptoms that I have been having for the last few weeks or so. There are severe aches and pains all over the body with profound malaise and a severe chill that has crippled me. There is no actual physical disease as such- I know that because I am a doctor by the way- but the distress it has caused me is immense. I have continued with my intensive practice and have not given up on it.I am hopeful that in near future, my meditations will help me to conquer this disability, yet I am worried at the moment. I seek the opinion and help of my senior brethren on the path to deal with this problem. Thanks.