Sensations from Sex

[Advice1]

Hi to forum members i am reasonably slim fit 50 yr old male normal BMI i exercise by brisk walking a few miles everyday.No medications or conditions but did have heart Surgery a few yrs back ,Mitral Valve Repair . i want to know When you have sex in a standing position being adventurous and excited in the moment with your Partner or be it just standing watching porn i sometimes notice heart racing quite fast in these standing positions and a sense of excitement totaly absorbed and have had strange temporary nauseous feeling like when you go cross eyed that has happened 3 times now (Ok there is bound to be someone making a joke about that cross eyed bit!)but it feels like that ,and feeling of nearly fainting but i do not faint .could it be to do with breathing or high spike in Blood pressure or got too over excited ,or possibly a problem related to heart or Lungs ,I have stopped this now as it worried me at the time but i am not overly worried just wondered what it could be caused by ,although i could say caused by myself!I have seen some studies from michigan university USA that in fact different to the norm i have heard that sex is healthy for the over 50 yr olds but they say that sex is bad for the health of the Over 50 male while it is good for the over 50 female due to the release of hormones after orgasm in females.

I would be interested in knowing what it could be

Thanks