I, m 31, had stress test/ecogram of heart 4 months ago, signs were great. Can I develop an aortic aneurysm since then? I'v having mid chest/ back pain

Yes. Anything can develop. The stress test was a moment in time. However, a stress test looks for blockages in the coronary arteries of your heart, not an aortic aneurysm. Your symptoms could be an aneurysm. If you have personal or family risk factors for that, you need that evaluated. That can be done by angiography or ct.