5 days a week
7 months (about 30 weeks)
16 miles per day
=============
About 2400 miles

That is well within the range of a chain wearing out, especially if it has seen significant use in the rain.

Measure your chain length. Typically 12", if it is 12 1/16, between pins, then it is time for a new chain. If it is 12 1/8", then it is past time for a new chain.