Steroids create alot of water gain and cardiovascular pressures are sometimes increased and this may create a little bit of edema ( leakage you may not be aware of ) into the lung slightly impairing gas exchange ( CO2 out and O2 in. ) It may also cause swelling of the soft tissue of tongue and throat going into you windpipe thus causing you to easily obstruct your airway. Apnea or the stopping of ventilation from the obstructing tongue and soft tissue of the airway when it occurs during sleep is " obstructive sleep apnea " Your problem whether a little edema in the lung or obstructive apnea if it is caused by fluid retention can probably easily get better with the use of something that gets you peeing off the fluid. It may take a couple days but you will offload. Coffee, ephedrine, lasix, most OTC diet pills will all probably get you peeing. If this is the problem, make sure you add some armidex or other drug that keeps water bloat from occuring in your cycle.
It is probably a slight elevation in your arterial CO2 ( carbon dioxide levels ) that is giving you the sensation you describe. Obviously a few slow deep breathes will blow off more CO2 and may eliminate the feeling momentarilly.
None of this takes the place of a physicians help. It is amazing how scary it gets when one gets that feeling and beaware that as frightening as it is, you do not have underlying lung disease. A mild anti anxiety drug will also get you over those episodes, But correcting what you have created may help. It is a side effect of steroids I believe as it is not uncommon to experience these sensations particularly at high doses. Flash pulmonary edema ( sudden and profound flooding of the airsacs of the lugs ) has killed a number of high level, high dosing, bodybuilders.
Just relax ( easier sad then done ) and know which ER you would use if you really do need help.
Allbeef