Take the back off and have a look at where the regulator is set, but it's unlikely.
Has the watch recently been serviced? It's possible that the incorrect oil was used on the balance staff pivots, possibly a dunk in Sololube or similar. Less drag on the pivots can cause the balance to swing too far, making the watch run slow.
Another possibility is that the balance and hairspring (or just the hairspring) have been swapped, and the hairspring is too long, or the wrong type.
It's also possible that the pin on the regulator was bent when the spring was removed for cleaning, and then not bent back on reassembly.
Although I've been repairing clocks for 15 years, I'm new to repairing watches, so there may be other causes.