Hi Maxx,
Heavy legs, muscles pain, fatigue could all be part of dehydration as well and not just the 'water' kind. When you are dehydrated you are also low on essential electrolytes such as potassium, calcium, sodium and the rest of their buddies...that can leave you feeling like each leg weighs 1000lbs and is impossible to drag up another step.
It can also cause irritability, cold sweats, general fatigue, weakness, joint pain, anxiety and headaches along with about a dozen other symptoms that I am forgetting.
If you check the box under at least 3 of the symptoms, start by upping your electrolyte intake...you may try drinking coconut water (I have found it useful although not everyone like the taste and beware of the sugar content...use all natural) 3xs/day...do not forget to drink a large glass of water at night before bed...We dehydrate at night too due to sweating and general bodily functions so you need to feed your body water then too. (Yes, I know, you hate getting up to pee at 3am but that is the price that we pay!)
In my opinion it is the first step...if you do not feel any relief with increased hydration and electrolytes then go to the next step of bloodwork to see if you have an underlying infection. They can make you feel 100yrs old and exhausted.
I alway try the easy fix first...it is cheaper and easier and makes me feel like I am in control. Somewhat.
Sharon