Sunday the 31st is this Sunday
Sunday the 7th Feb is next Sunday
Simples
Well it is now cus it's tomorrow, today being Saturday like.
A week tomorrow is therefore NEXT Sunday providing you use that full sentence in its full context.
As BM very correctly points out, NEXT week starts NEXT Monday, which is the following Monday after the last one, i.e. the 31st.
What on earth is so difficult to understand about this ey, what...exactly?