I'm obese(bmi=41).i felt dispnea during light exercise(walking), but now also during rest. i have periods with tachycardia.is it due to my obesity?

Tachycardia: it is not normal to have tachycardia at rest. tachycardia means heart rate more than 100. if that's the case you need to consult your physician asap.

It could be but ...: Make sure you review these symptoms urgently with your doctor. As depending on your level of fitness, it may be deconditioning and weight related but also could be a more concerning medical condition that may be developing. Please be careful about downplaying symptoms like shortness of breath or tachycardia. Best of luck.