Q: macbook heating up

hi there

My macbook pro is a little old now, its a 2011 model, im still running snow leopard that came with it, I havent upgraded it or changed it in any way, its still 4gb of ram

Anyway, I am doing some heavy video editing with Final Cut Pro X using an external HD with all my files on it (usb 2.0) and my mac struggles a bit

One observation that I have noticed that I am finding slightly worrying is that when I am editing heavily and I can hear the fan working hard, my mac is getting very hot over where the power chord is plugged in (I usually always work with the chord in since the mac is working hard) - but its getting really really hot, I think it never used to do this? - usually the whole left hand top corner of the mac is really really hot- when it gets really bad i usually stop editing or close down the mac

sometimes I get the 'Service Battery' message when the battery is charging- could this be something to do with it?

other things to consider, my original charger broke and I bought a third party one, recently the cable has frayed and im worried there is something wrong with- its usually when the external HD is plugged in and when the charger is plugged in too that the macbook heats up!

Any thoughts?

should i be scared that my mac is heating up so much?

thanks