Lightheaded - could it be the heat?

I just had my labs done last week, and everything was normal: iron, B12, protein, everything.  However, for the last several days, I have been experiencing occasional lightheadedness, mostly when I stand up quickly after sitting for a while, but sometimes when I exert myself like carrying something heavy up the stairs. 

We have been having what passes for hot weather here in western Washington (temps in the 80's), and while the heat doesn't bother me temperature-wise like it used to (I'm not sweating, but everyone else is instead of the other way around), I think it might be the heat that is bothering me.  I don't have this problem at work, where it is air-conditioned, just at home where it is not. 

I have been trying to drink more water, but getting enough fluids has always been a challenge for me, so while my consumption is up, it is still only about 48 ounces a day.  Having said that, I normally drink more like 32 ounces a day with no noticeable problems - I know, I know, it's not enough, but I am just being honest.

Any thoughts?

 Are you taking BP meds?

I have been off my BP meds since I was 3 months out from surgery.  My last BP was taken last week and it was 102/72, so I think I am good there.  It wouldn't hurt to take it again, though, so I will go do that.  We have a BP machine here at work.  Thanks for the thought!

I think it's dehydration. I'm not getting on you at all for not getting enough fluids. I know it can be tough.

And it could be the heat ( I use that relatively. I'm in W. Wash. too). Some folks are sensitvive to small temperature changes.

Is your fluid intake trouble with water specifically, or just with volume?

You may be slightly dehydrated due to the heat.  The same thing happened to me about a month ago.  I was mowing the yard and felt light headed and my heart was racing.  I quit for the night.  The next day I made sure I had lots of fluids in me and it made all the difference.  I find I get dehydrated much quicker now.

It could be dehydration. We were also told at one of our classes that we would get positional dizziness, where if we stood up to fast or over exerted ourselves, we would get dizzy, but that it is normal.
Either way, I'd try drinking more water.

probably dehydration. even drinking almost 100 ounces almost every day i still just got back from the ER for that. take it serious!

Thanks for all the replies!  I took my BP and it was 110/75, so I am thinking it is the heat and dehydration.  I will work on getting more liquids in.