aww bless you! im only 13 weeks and i feel rough as anything, out of breath and just plain knackered all the time so i can sympathise!!!!
have you thought about going on your maternity early??
id say get a sick note off the doctor but you will prob find that your work could actually start your maternity early anyway,
i dont want to start mine early but if i feel ill i would have to, you need to think about your health, when the little bundle of joy comes, you will be even more tired!!
you are entitled as far as i know to take your maternity 11 weeks before the baby is due, so you could actually finish now or in a weeks time,
good luck, take care xxx