Working on my treadmill makes my legs cramp.

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I'm finding it really hard to do the treadmill. I'm okay with it before but now I can't seem to even make it to 10 minutes. A couple of years ago, my legs cramp every now and then. I think it's whenever I stretch sometimes or stand on tip toes, like when reaching for something high. I'm not really sure if the problem are my legs or I'm just doing it the wrong way. I tried to do interval training on the treadmill. Interval training is where you switch speeds with same duration (ex. walk briskly for 2 minutes then run for 2 minutes, then back to brisk walking for 2 minutes. can also be done on the stationery bike). I can easily do the bike but not the treadmill. It makes my legs cramp like hell! I don't know what my problem is, since I was okay with it before. My dad said I probably lack potassium and that makes my legs cramp easily. I've put on a lot of weight. Is it because my problem with my legs? Am I doing it wrong? Or Is it normal for someone who put on a lot of weight and haven't had exercise for a very long time? Can anyone please help me? Tips/techniques would be very much appreciated. Thanks!