If you are using a panty liner only - not a full pad, and you don't have to change it constantly - then this is not about blood loss. I don't know what is causing your light-headed feeling, but it's not b/c of blood.
If, as someone mentioned below, you ARE using full pads and changing them every hour or two because there is so much blood, then that's a totally different story. But your description doesn't sound like this.
Also, after I had Mirena inserted, I had spotting every day for several months. I only had to use a panty liner, and that liner lasted the entire day. But my doctor said that light bleeding was totally normal for the first 3 months (and even up to 6 months its considered normal as long as it's not a lot of blood). It sounds like you are in that timeframe and you aren't bleeding excessively, so that part sounds ok.
Of course you should still follow up with the light-headed nauseaous feeling with your doctor. But I would not necessarily blame it on your Mirena.