Breathing after a strained pectoral muscle can be painful because the chest expands when you inhale deeply, which pulls on the pectoral muscles and causes pain. Try not to do chest expansion exercises or take deep big breaths after a pulled pectoral muscle, especially if you can cover your chest when coughing to minimize pain. If the pectoral muscle is strained, non-steroidal anti-inflammatory drugs can be given to treat it, such as Fenpropidol, and these should always be taken with a meal as there may be signs of irritation to the stomach. You can also apply a topical application of Fitalin or use Yunnan Baiyao aerosol and spray it on the strained pectoral muscle to provide localized pain relief. For a strained pectoral muscle it will take at least a week for the strain to gradually resolve, so try not to carry out the above activities in the meantime as they may cause the strain to worsen.