One of your turning light's bulb is burn, and need to replace. It is a simple bulb replacement, which cost $5 - $10.
Mechanic may charge $10 - $25.
The flashing relay has a build in timer circuit that will double (approximate) the flashing frequency when the other light bulb is burn, or disconnected.
Brake light, and headlight doesn't has flashing mechanism, so either your car has other warning light to tell you so, or you have to examine yourself by turning each one on, and pressing on the brake padel.
Hope this help