I think it's pretty normal. Changing body weight can affect how the hormones in your pills work with your body.
If you're really concerned, you could probably call your doctor and talk tot he nurse practitioner. I suspect, though, they'd tell you that spotting is normal on the pill - even if you've never had it previously - and that you don't need to worry unless it becomes a flow.
(Disclaimer: I'm not a doctor or a nurse or any kind of medical professional - nor do I play one on TV!
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