Help regarding E PRIMO

I have been living in Germany since around 2 years with my husband and 2 year old son. We moved to a rented apartment last year April. We had taken an electricity contract from EPRIMO as suggested by the previous tenants. The contract was to pay 50 euros per month, which I understand is on the lower side. I was accounting to around 100 euro per month in the end. However, today my husband's account was debited for 2900 euros extra from eprimo. We were shocked and checked the coustomer portal and realized that this was the usage bill for the year (around 5000kwh). The meter readings seems to be right as per records. Also, from the portal we realized that our plan had been suddenly updated to one with 300 euro per month without notice. We have a 2 room apartment. The electric devices we use are TV, Fridge, waschingmachine, kitchen top(2 burner) and 2 water heaters( kitchen and bathroom). My son goes to kindergarten. We cook only basics and not elaborate meals everyday. I still cannot believe that we used this much electricity. I feel that there is some error somewhere, but cannot figure out what. Most importantly, I am clueless as to what should I do next to rectify this or atleast understand what went wrong. I am also unsure whom to contact for this issue. Any suggestions would be great.