Have I gained weight too fast?

My sister is a gym freak, I'm not, and it's starting to show over the last 5 months or so. She explained she was worried about my weight, and I admitted I was worried at how fast my belly was growing. I hopped on the scales in front of her and it read 208ibs, my face went bright red and she said "you have got a lot bigger haven't you" I felt ashamed. I was 167ibs 5 months ago, meaning I've gained 41ibs no wonder I'm so large. She is now trying to get me on a diet. She says this is way too much to gain so quick is she right?
Have I gained weight too fast?
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