carlie,
just because youre 8 stone makes no difference. Your body will now be feeling the extra weight and pressure of your baby on your lungs etc.
If baby isnt fully engaged then it will feel more uncomfortable, you are also carrying more blood, to supply you and baby, so that in turn makes you feel hotter, you heart may be pumping harder too.
BUT have a word with your midwife, thats what are there for, and tell her how you are feeling. Im sure that many women feel like this in the last few weeks, i know i did.