Sounds like the Penicillin has worked a treat. I am allergic to the stuff, unfortunately, quite severely.
I am sure that it is the penicillin that has worked its magic for you Tilly, especially if you were suffering from a bacterial URTI as Sqad describes, but its also possible that the placebo effect from the "rituals of care" of going to your GP and having a discussion with a (hopefully) friendly, professional and sympathetic medical person and the knowledge of something being done contributes to your recovery.
And there is also the possibility of the contribution of "regression to the mean". That is the understanding that most colds and infections tend to be self-limiting - that is, there is a natural span of time for the infection to run its course and for you to return to feeling good/normal again, and it might just be that your cold had more or less run its course anyway.