you should have started searching for jobs/OPTs 3-4 months before you graduated. it'll take 2-3 months for the institution to sponsor you as their intern/assistant, or whatever. this is the case for most of the students doing biomedical sciences. i'm not sure how it exactly works for nursing, i don't think it's that different.
also, talk to medical colleges in houston, they collaborate with a lot of hospitals in the area and might be able to place you somewhere.
good luck!