I think if at all possible you need to request to see a different gp. Before you go, make a list of everything you are experiencing so you can make sure you don't miss anything when you tell the doctor.

It is important to remember that it is possible that your symptoms are all unrelated issues. See another doctor and ask them to check your lump. If they think it is anything to slightly be concerned about, they will send you for a scan and possibly a biopsy. This will be able to tell you once and for all if it is a lump to worry about. During the scan they will also be very thorough and check for any other lumps in the area, so you can be sure if there is anything, they will catch it. How long have you had the lump for? 

The other symptoms may be something else entirely. Stress and anxiety attacks can cause severe headaches and breathing/ swallowing difficulties, so that is something else to consider. Migraines can also cause other seemingly unrelated symptoms in addition to constant and severe headaches.

As hard as it feels, you need to try and keep yourself calm. Go to the doctors and insist on seeing another gp for a second opinion, you might want to try and book a double appointment so you have time to discuss everything with them. Tell them about the lump, then tell them about the other problems, and ask them to go through each of them with you. Hopefully they will be able to run some tests and put your mind at ease, or refer you to a breast clinic for further scans.