Blocked ear and tinnitus. Heavy weights?

I am experiencing loss of hearing in my left ear and also some tinnitus. This started couple of days back when I was doing cardio exercise with my mp3 player plugged in my ears. Once I stepped off of the treadmill, I sort of started feeling my left ear is blocked (like it happens when under water or during a bad cold). Strangely, the problem happens only when my head is in horizontal position, i.e. when I'm looking forward. It goes away when I look up or look down. It also goes away after a night's sleep and comes back during mid day and stays till I go to bed at night. The loss of hearing is significant as I'm having trouble listening to a person sitting in front of me in meeting rooms etc. Please advise what could be causing this. I work out regularly - both cardio as well as free weights. Is this happening due to excessive pressure within the skull when lifting heavy weights? I have stopped working out for the past 2 days thinking its happening due to strain on my ageing body.