I have BPH and am taking flowmax which working fine I also use a cock ring from time to time. Could this effect my bph?

Unrelated. Flomax (tamsulosin) is an excellent treatment for symptoms of bph. A penile ring is an accepted but somewhat dangerous sexual aid, especially if there is venous leak and erectile dysfunction. They are independent and unrelated.
No. Flomax (tamsulosin) has direct effect on the prostate gland so urination does not become obstructed. Sexual function is different as blood flow is required and any instrumentation that blocks this can be a problem.
No,... Congratulate for getting help in urination from taking flomax; so, please keep on taking it indefinitely. But, in time, sooner or later, you may find Flomax (tamsulosin) becoming not good enough for you, largely because the accumulation of undesirable ill effect of aging is exceeding what Flomax (tamsulosin) can do for you. As to cock ring use for ed, it bears no negative effect to bph.