Small losses of credit are usually caused by small amounts of data being used without an active internet inclusive goodybag. It's not a glitch on giffgaff's part.
You lost credit due to either an app using data or straying from the giffgaff top up page.
Please contact an agent and they will refund you enough this time to purchase a goodybag. Go HERE
Please contact an agent and they will refund you here link
hopefully they will get onto your problem as soon as possible
for replies from agents
A few suggestions to prevent this happening in future: