Starter Problem

Jumper the relay by jumpering the two large bare terminals.

I would start by clipping your voltmeter right to the main large stud on the starter. Jumper around the relay and observe what happens. Does the starter "clunk" once but not crank? Does it "buzz?" And WHILE you have the starter engaged, observe the voltage reading and post back.

If the voltage is very low, the problem is in your cables and terminals

If the voltage is close to battery, the problem is in the starter, and likely just happen-chance that it occured with your relay mishap