gestational diabetes/thyroid problem? help please

I'm 20 weeks pregnant tomorrow with my third baby and heavier than I was with my previous two babies. The last 3/4 weeks I have been feeling off, can't really put my finger on it. I feel reallly lethargic all the time, even after 9/10 hours sleep! And I always feel super sleepy after lunch, particularly if it is a carby lunch. And most afternoons around 3/4pm I get a headache! I spoke to a midwife on the phone who said it could be my thyroid or gestational diabetes. Next week I'm having the blood test to check thyroid and on 17th November I'm having the glucose tolerance blood test, 3 weeks earlier than I should have had it. I'm just wondering if anyone Has had experience of either conditions in pregnancy and do my symptoms sound similar to yours?? Thanks in advance for any comments/advice. I'm a worried mummy :(