Your symptoms are not likely due to blood clot. This is also not likely to be related to the heart; with ECG being normal and no other risk factor. This is likely to be;
1) Costochondritis; it is inflammation at the junctions of rib and cartilage or with the breast bone / sternum. Local analgesic ointment, or spray, chest brace and warm compresses with Ibuprofen are helpful.
2) Another possibility is muscle sprain.
Please read about the costochondritis;
Your anxiety may also be responsible for the aggravation of the pain.
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.
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