Firstly you need to determine whether your speedometer is accurate or not. Ask a friend to drive and hold different speeds in a car with an accurate speedometer and then pace behind that car to see how yours compares. If your car's speedo reading is different then you know you have an issue.
Speedometers work by measuring the rotation speed of something that spins and translating that to a speed estimate. The sensors can be mechanical or electrical and are usually mounted on a wheel or the differential. Some changes can make your speedometer inaccurate, for instance if you change your tire size or differential ratios (presuming the speedometer is measured in the diff). If you put on larger tires your speedometer will under-read, i.e. you're going faster than you think, if you put on smaller tires it will over-read. Replacing or adjusting wheel speed sensors could cause issues as well if your car uses the ABS wheel speed sensors for speedometer readings.
However, changing the oil on your car and doing regular service items should make no difference whatsoever to the speedometer reading. Changing to tires with less grip, changing the alignment or messing with the steering geometry, changing shocks and springs and other things can radically change how a car handles. If you feel that your handling is worse after you did some work look there.
It's also possible it's your perception or your driving habits have changed since you worked on your car. If your car's more peppy you might be driving faster and entering a curve with more speed than you used to, making you slow down in the curve to stay on the road.