I would call and let them know this week, they'll probably have you come in for blood and urine work... extreme fatigue is a symtom of many problems, anemia & blood sugar issues (either too high OR too low) leap to mind. Both are easily fixed.
Same token there is a VERY small subset of women who instead of POST partum depression, hormone trigger depression while pregnant. In fact, for those of us who are lucky enough to get it (actually, that's only 1/2 sarcastic... because the MOMENT your child is born you start feeling *wonderful* / more like yourself more and more every day, which REALLY helps with the whole sleep dep thing)... there is something called "the suicide hours". Usually in the evening/ late afternoon... lasts typically for 1-2 hours. In the beginning it starts out as an "I can't DO this anymore, I'm EXHAUSTED." and by 2nd trimester you're sitting on a chair gripping it with both hands to keep from offing yourself. The ONLY nice thing about "the suicide hours" is that you can a) plan for them and b) you really gain a lot of faith in knowing they will pass. HOWEVER... there are a handful of antidepressants that are totally safe to take while pregnant. Sigh. Didn't learn about them until after my son was born and I was chatting up with my OB telling her what a *relief* it was. ((She struck her hand to her forehead and said "HONEY... these are things you TELL your doctor. We could have fixed you up very very simply on either a, b, or c." Whoops.)) Pregnancy depression runs in my family, however, so it didn't even occur to me to mention it. Just shared a lot of macabre humor with the other moms in my family and suffered through, and slept as much as I could. Sigh.