Starting your period means nothing, Gina. It is very common to have a period while ebf - plenty of women find it starts about now (and some even earlier). You increase your chances of not having your period come back if your bf is very frequent - 10-12 or more times round the clock, inc at night. But even then, some women start just weeks after birth. Forget about it - it's not significant.
It's true that some babies seem to fuss more just before a period - I cant remember the reason for this, but IIRC it's something to do with the taste of the milk.
Diagnosing dehydration from a sunken fontanelle is really hard in babies, whose soft spot does not close up anyway. There are always other signs anyway - listlessness, lethargy, sunken eyes, dry mouth.
As ever, if you are worried about your baby's health, you need to speak to a doctor or HV.
Why bother expressing? I can't see the need for it and it's a hassle :)
What do you think of my suggestion to get help for your anxiety rather than your breastfeeding (which is ok!)?