Airtel — Internet speed dropped dramatically since 3 months

I am an airtel customer since 11 years. I have no complaints till now. But, since the last 5 months my internet speeds are 'pathetic'. I always use the phone on my bed after coming back from my work, suddenly my internet became slow one day and since then it remained slow. My download speeds are not more than 0.4 mbps now, 5 months back the download speed was approx 6 mbps. I don't know what went wrong.