Thank you for your question. Acne on the back or trunk is not uncommon. Some people get acne more on their back with little involvement of the face. The cause of acne on trunk is similar to acne on the face, as the trunk also contains a lot of sebaceous glands. Hormonal factors do play an important part.
I do not think that changes in bedsheets will lead to truncal acne. It is difficult to say what has triggered your acne.
The treatment of truncal acne is quite similar to treatment of acne on the face. Topical medications like benzoyl peroxide, anitbiotic lotion or gels, topical tretnoin creams can be used to treat both acne on the face and trunk. In more severe cases, oral antibiotics like doxycyline or isotretinoin can be used.
In general, I do find that truncal acne responds more slowly than facial acne to treatment and can be more difficult to treat.
If persistent despite the above treatments, other causes of acne-like rashes like pityrosporom folliculitis ( a type of fungal folliculitis) has to considered, and a visit to doctor/dermatolgist should be considered.