Hi everyone, i was wondering why am i always (almost) blacking out when i ride a bike. I'm female and 21 years old.  When i was younger, i never had this kind of problem. Like when i was 13, i need to run for miles before i felt like I'm going to black out, and i just felt it occasionally so i didn't think too much of it. But when I'm getting older, it's getting worse. Last year, i was out biking with my friend. I only last for 40 minutes or so before i felt like I'm going to pass out. The process was slow, first i had a terrible headache, nausea and my vision began to deteriorate. After that, it took my hearing as well. I was barely able to walk. Luckily, after i lied down for around 5 minutes, they were gone. I just thought it was because i was out of shape. I rarely exercise after i busted my knee in high school after all. and i just kind of gave up and swore off any biking activities. But This year, i moved to a new place and think that i need to invest in a bike to safe bus fare. i tried to walk a few km to check whether my stamina is really that bad. I walked 3.6 km nonstop with no problem. And a few days later i tried to walk for around 4 km, and took a path that had a lot of hills and slope,  i was also carrying my bag that had my laptop and tons of books in it. It makes me tired, but that was it, no dizzy spell or anything. yesterday, i tried to ride again and bike for around 17 minutes. I felt a bit dizzy near the end of the ride but i ignored it. After i parked my bike suddenly i felt like my head was hit by a brick, it felt just like my old episodes but it last longer and for some reason, as time pass, it was more harder for me to breathe, my chest felt tight and my heart was going crazy. I tried to check my heart rate and unfortunately, my fitbit couldn't read the heart beat (it showed -- bpm). Even though I sit and rest for a good 20 minutes or so, it was really hard for me to reach back to my apartment, i stopped 3 times just to catch my breath and i walked like a person with bad hangover. 

Now the question is, do i need to see a doctor? Or is this because I'm just really out of shape? I am a bit worried because my grandma has a really weak heart, and me being her grandchild could inherit it.  Any advise/help is appreciated and i hope my writing is not that confusing, I'm not a native speaker :)

Thank you!!