Breastfeeding and pregnancy?

Don’t need any judgement, just looking for any one with first hand experience or knowledge!

Full time breastfeeding, with the occasional bottle of pumped milk or formula, for my four month old (was due early December, but baby was born at 35 weeks). No period yet.

Worried that I may become pregnant after our pull out method was unsuccessful. Took an ovulation strip test, and test line was almost as dark as control line. Without knowing cycle day (no cycle yet), when should I start testing for pregnancy?

Appreciate any advice!