I really doubt you will get stretch marks. Stretch marks are from rapidly gaining weight - not loosing weight.
When you rapidly gain weight and the skin isn't able to expand fast enough, stretch marks occur.
As for how not to loose gains from a steroid cycle -
Not keeping the majority of the gains you made on your steroid cycle is usually the result of:
1. Not running a proper PCT after your steroid cycle.
2. Not training and eating with the same intensity after the cycle as you did on cycle.
What put the muscle on the first place is what will keep it there after your PCT - HARD training and a high protein diet.
As a female, you will want to use a good natural testosterone booster like HCGenerate ES from n2bm.com as a mini PCT after every AAS cycle you do.