Tennis and vision/hearing loss?

So I've been playing a LOT of tennis lately, and it's been severely hot. (90 degrees or higher) The past few times I have played with someone better than me I noticed I start to sweat excessively and quickly, and after maybe 40 minutes, if I slow down for a moment, my vision starts to go black and I feel like I cannot go on. So I sit down for a few minutes and drink water, but sometimes sitting makes my hearing get really fuzzy.
I've been playing tennis for the past 3 summers and never had this problem. My diet has changed this year, however, and I am consuming a lot less sugar. My mom and grandma both have heart failure and my dad has heat strokes. I'm worried that this is a sign that I have a heart condition, or if it's normal? I also run and do not get this problem, but I run at night.

 

Maybe you are having heat strokes? Consult your doctor not hipforums.

 

No it's not that severe. I would if I had money/insurance/a doctor.