Brief Answer:
Detailed proper history and clinical examination by a General Surgeon.

Detailed Answer:
Hi, thanks for your inquiry.

To recapitulate:
Young Female/24 - Lump in the middle of lower right *chest area - small pain under right *breastbone - shortness of breath for 2 weeks - need X-ray, but said fine - needs ultrasonography but takes a month for appointment - worried about the cause.

I can understand the plight you must be going through.

To make the history more clear for me to understand and assist you more please give some more details.
- Since when is the lump present?
- Is the lump in the breast tissue or below it?
- Is the lump freely XXXXXXX or less XXXXXXX or fixed?
- is it tender (pain on pressure)?

- Does the pain under the right breast bone means to the right of the breastbone (meaning sternum). Since when is the pain? Is it intermittent or continuous? Is it different than the lump area?

- Do you mean to say that the X-ray was not done?
- I hope you have taken an appointment for the ultrasonography.

My thoughts:
Since you have a lump, have an area of pain and also have breathlessness (for 2 weeks), a good clinical evaluation a proper clinical examination by an expert Doctor like a General Surgeon can tell you on these grounds the possible causes of all these.
-If the auscultation by a Doctor gives absolutely normal sounds, X-ray may not be done; He may be feeling that the cause of shortness of breath can be due to pain you have or due to anxiety (the causes). If there is no other associated history of fever, expectoration, chills or so your Doctor would definitely associate/correlate with the history you have given and the findings he/she has found on clinical examination.
-What was the opinion of your Doctor on examination- about hte lump, the area of pain and cause of shortness of breath.

-The lump: if in the breast and if you can move it freely in any direction without any pain, it can be a non-cancerous lump and considering your age it can be adenoma.
If the lump is not easily movable, is painful on pressure and if you can not go around it, it can be a fibrocystic disease- again a non-cancerous condition.
If it is not in the breast- it can be a lipoma or a sebaceous cyst.

-The Pain: if it is on the right side of the breastbone, the commonest cause is costochondritis. This is again a clinical diagnosis on good history and local examination by a Doctor.

Considering your age, you can wait for 1 month.
Try for an appointment with a General Surgeon as clinical examination can give you the diagnosis and the tests of ultrasonography, mammography, Fine needle aspiration cytology or biopsy are all done just for confirmation whether this is a cancer or not cancer.

I hope this answer helps you to clear your doubts.
Please feel free to ask further relevant queries after carefully reading the detailed answer I have given or if you feel that there is a gap-of-communications.

---

Brief Answer:
Costochondritis.

Detailed Answer:
Thanks for your feedback.
The pain you have described looks to be due to costochondritis.
Take anti-inflammatory like Ibuprofen, which I hope your Doctor must have given you.
There is a role of anxiety and stress as these can increase such pains.
OK- get the ultrasonography done and get rest of the things as explained above.

---