Is it normal to lose and gain weight like this?

For me, I can go anywhere from 130-140 in a month depending on my diet, stress level, time of month and so on. I have a slightly hyperactive thyroid but I'm otherwise healthy, so if I reduce ny caloric intake for a few days I can lose up to 3lbs in those few days only to gain it back after a couple of good meals.

Bear in mind that your body weight includes water, the food you've even recently, fecal matter and so on, not just organ, bone, muscle and body fat so of course there will be a variable. If you're truly worried about your health, have a doctor check you and your cholesterol. If you haven't done so lately then you should.probably do so soon. Could be nothing, could be something.

You lose so much weight per day. People will lose this much from things like going to the bathroom and food being digested. If it is a small amount of weight you fluctuate between it is normal. However if it is a large amount it means something is wrong and you should probobly consult a professional on the matter.