Gained a few pounds... water weight or fat??

For the past few months I've been consistent in weighing between 114-116 pounds (113.5 if I check in am, 116.5 if I check during the day).  For the past week, I've noticed that I have been bloated and my weight is around 115-118.  I should be getting my period next week (my body is still getting on a schedule).  I'm assuming it's just water weight with how it came out of nowhere and because my stomach is hard as rock - its not "flabby" like fat gain.  I'm more or less wondering if there is a way to tell if it's water weight or not, also how much can you fluctuate.  I've heard if you take your average weight you can gan up to 2-7 additional pounds.  Is this true?

 Also I have been recently seeing a kidney specialist for possible kidney disease or nephritis.  Could kidney issues cause additional bloating?

Yes, it's true that weight increases prior to a period can be up to 7 lbs. There's no tried and true way to tell if it's water, but generally that bloated feeling is a good indicator.

It is most likely due to your normal hormonal fluctuations however as you are beginning to investigate kidney disease with a specialist it would be a good idea to let him or her know that although your period is due in a week, you just wanted to flag that you have some noticeable bloating at the moment.

I'd guess the bloating is hormonal.  But it could be related to a few other fairly common things.

• eating lots of high-sodium foods, which in excess can make you retain fluids
• are you constipated at all?  Not peeing as much?  I mean, what goes in must come out lol.  If you aren't regular the food, fluids, etc are just sitting in there, and that adds weight.  And if your stomach is hard, that can often indicate being "backed up".  That's what it sounds like to me.  
• I don't know that much about kidneys, but they help filter bad things out of your body, right?  So if you had a lot of sodium or something, and your kidney's aren't working right, that might make it more likely for you to retain water.  But, again, I dont know what the issue with your kidneys is. 

I wouldn't worry about the gain though.  The pattern ofyour weight is more important/the general trend of your weight.  

This doesn't work for everybody, but I've found that having a tall glass of water with about 2 tbsp of apple cider vinegar brings down my pms bloating considerably.