It very much depends on the rate and type of "flicking". Slow, leisurely "swishing" is often a sign of contentment, while faster, more agitated flicking is excitement. Whether the excitement is anger or pent-up energy is largely dependent on context and other body language - eg. a playing cat will flick its tail, but its ears are up and forward, and it's fur is flat, so it's excited and enjoying itself. An angry cat also flicks its tail, but it's ears are usually back or flat against the head, and it's likely to growl and arch the back with fur standing on end. No one aspect of body language tells the whole story - you need to look at the whole body posture, behaviour, context, vocalisation, personality of the cat, other animals in the environment, etc.
Based on your description, it sound like she is fairly happy, going by otherwise relaxed body posture and purring, so she is either doing a content tail swish, or if its more flicking, she is just excited or has some pent up energy
Any pics of this lovely lady?