I am sorry to hear you have these concerns regarding Duchess.
There are a few things that could be going on. DO you know how long she has been like this? If it is a short period of time, (less than 24 hours), then the most likely thing is an allergic reaction to something. We quite often see these types of symptoms in the summer with bee or wasp stings. These can often go down as quickly as they appear, but you can try giving her either cetirazine (an antihistamine availble from the chemist) - just one 10mg tablet, or a piriton tablet (one 4mg every 8 hours). The cetirazine is better if you can get that.
If the swelling has been present for longer than 24 hours and is not going down then I would be more concerned about some type of infection. At this point I would suggest a vet appointment is the best option as then she can be fully checked over to try and ID a source for the infection - is it teeth, lymph nodes or an abscess for example.
The only other I would add at this point is to keep an eye on her breathing - if the swelling either expnds up to her nose or back down her throat it might make it difficult for her to breath, in which case I would suggest an urgent trip to the vets.
If you can get back to me with any other information you may have, I can help you further.