Dot,

A AFCI is a device (circuit breaker) that is designed to detect arc faults,
and is superior protection against overcurrent or arcing problems. To my
knowledge, all four US manufacturers that make AFCI breakers incorporate
GFCI (ground fault) protection circuitry into their AFCI devices.

Code requires AFCI protection in bedrooms on new construction. The logic was
that this is an area of the home where elderly, disabled or young people
spend a lot of time.

Unlike a standard GFCI, an AFCI device will not trip on ground fault until
the faulting load is connected. Nuisance tripping (something like you
described here) often occurs when you plug in something new or turn a switch
on. The device will not trip again until the load is re-energized (turned
on).

The breaker could have tripped for any number of reasons. The advice I
always give people is to accept one trip, that will promptly reset, as a
fluke. More than one.... more investigation needs to be done. It could have
tripped on a ground fault OR a arc fault. Both are dangerous conditions if
they continue.

Have you recently plugged anything new into this circuit, or changed/moved
something around that is plugged into it? That's a good place to start
looking...

Jake