This is integer overflow. Once the maximum integer number is hit, it will jump to the smallest:
julia> typemax(Int)
9223372036854775807
julia> typemax(Int)+1
-9223372036854775808
You can avoid this by using floats or BigInt.
There have been quite a few posts on this on this forum, search them for more details.
PS: please quote your code, see Please read: make it easier to help you