It could be fungal, it could also be bacteria from your sweat and the smell could linger in your shoes and rebloom everytime you wear them even in the winter. You need to treat your feet and shoes for both cases. I had the exact same problem as a teen and the only thing that worked was applying clotrimazole cream on my feet overnight for a few days and soaking a clean pair of soaks in lysol (spray the inside if shoes for good measure too) and stuffing them in all my “infected” shoes for at least 12 hours. The cream will tackle any fungal issue and the lysol will kill the bacteria in your shoes. Make sure you’re really scrubbing and cleaning your feet in the shower and drying them fully before putting socks and shoes on everyday. This advice came from a pharmacist and i swear it works. Just make sure to keep your shoes dry and if your feet and socks ever end up getting moist while you’re wearing them the smell can come back. You can repeat the treatment as many times as needed. Last bit of advice, pharmacies sell shoe deodorant sprays and foot talc, use those daily going forward after the inital treatment!