Why do I have a stench?!

I hope she's saying it in a supportive way! As long as you shower at least once a day, wear an anti-perspirant (stops you sweating) rather than a deodorant (just covers the smell of sweat), and don't use strongly fragrances 'feminine cleansers' for your pubic areas (as this can alter the natural bacterias which can then cause problems such as thrush and bacterial vaginosis, both which can cause smells) you should smell fine. Also wearing cotton underwear is better rather than man made fabrics which can cause sweating. A bad smell coming from the mouth can indicate tummy trouble, if you're mouth hygiene is good (brushing twice daily and regular flossing) which will need a trip to your doctor to sort out. Hope thic helps.
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Can she define the odor? Is it BO? Dirty Hair? Bad breath? Vaginal discharge? Too much perfume? Dirty clothes? Drugs and alcohol? She needs to give you a few hints. Ask her for a little help and some suggestions.

Try changing the sheets on your bed, taking a shower and washing your hair with a nice shampoo and conditioner. Launder your clothes and vacuum the floor to make sure that isn't the cause. Personal hygiene is imperative. Make sure you brush your teeth daily. If you have cavities you might consider a mouthwash until you see a dentist. Use a good antiperspirant after your shower.

Avoid foods like cabbage, beans, and onions. These can cause body odor. Until you know for sure what smell she is complaining about. Then maybe add one food back at a time, one per week, and see if you can identify which it is.

Marijuana will leave you, your body, and your clothes in a cloud of vapor. You may not smell it, but other people who don't smoke it are more attuned to it. Heavy alcohol consumption can leave the odor exuding from your skin, even if you shower.

If you take steps to eliminate the possible odors, see your doctor. There may be a medical problem that needs intervention.

As for people not liking you any more, hmmm. If they were friends, surely someone would have said. If they hang with you now, and you take steps to eliminate whatever odor your mom is talking about, that won't be an issue.

Go back and talk to mom.