I'm 33 weeks plus 3 days and been know to preterm births for last couple months I've been badly constipated all of a sudden I get diahrrea and low pressure when walking I just wanted to know if there a chance labour could be approachin

Some women do have a bit of diahrrea before labor starts or in very early labor, but it certainly can be just from stomach upset. Low pressure, especially with walking, is pretty common towards the end of pregnancy.

Certainly with a history of preterm births, it would be wise to keep your midwife or doctor informed of any possible changes. They would much rather you be checked and it be nothing than to have you dismiss something.
If it continues, check in with them. If you notice any cramping that comes and goes... like contractions, let them know.