Fertility issues after having babies???

Has anyone ever experienced fertility issues after naturally having other babies? Is this a thing? We’ve been trying for another baby for 1.5 years. My doctor offered fertility testing in October and I declined because I thought surely if I’ve had other babies there’s no issue.

First and second baby: surprises

Third baby conceived first month trying.

So nothing in that tell me we’d struggle? Maybe we aren’t meant to have more kids.... or maybe something is off now? Is testing worth it if I’ve had kids before?