Hi,
A photo tells so much more. It shows the reality.
Text and sketches are influenced by your knowledge (that we don't have), your expectations .....
We still don't know what measurement device you use, which measurement tool setup, cable length.
You know what power supply you used, whether it is fixed or adjustable, how precise it is and how much noise you can expect.
You know whether you really did a measurement at the wires of the sensor, or you did a measurement of the supply without the sensor connected, or you just expect it to be 3.3V.
We don't know this.
Mesuring millivolts is simple as long as you know what you do. People here in the forum have quite a huge range of experience. From absolute beginner to absolute expert.
I guess ( and that's all I can do) that you know that there is a huge difference doing a true RMS measurement (including all noise and mains hum) versus duing the same measurement but in "average mode" (exluding most of the noise).
And so on. One could write a book about correct and accurate strain gauge measurements and the pitfalls.
Klaus