If you sew a couple of pieces of fabric together (not strips) without guiding with your hands, you get the curve? If so, this usually means that the feed dogs are not aligned correctly. If they are off by a few degrees, the machine will sew a curve rather than a straight line. My Bernina will sew a straight line even if my hands do not guide the fabric.
Since your machine is only a couple of months old, it is still under warranty. I would take a sample of the sewing and the machine back to where you bought it and ask them to correct the problem. When you pick up the machine, bring some fabric with you and do a test sew to make sure that they actually corrected the problem; you don't want to get the machine all the way home only to discover that the machine still does the same thing.
If the machine sews a straight line on fabric but your thin strips still end up curved, this indicates that the presser foot pressure might be too tight. Some machines allow you to adjust presser foot pressure; check your manual. If the pressure is too high, the foot is pressing on the fabric edges so tightly the edges are stretching as they are sewn. This results in a curved strip after sewing.
In general, high quality machines will come out of the factory with well-aligned feed dogs and presser foot pressure correct. I am not an expert but, in my limited experience, Necchi is not a great brand anymore. Vintage Necchi's are collector's items, but I'm afraid the Necchi brand has gone much the way of the Singer brand in terms of quality in sewing machines. It's not only the way they come out of the factory, either. It's also in how they hold their adjustments. You may want to look into "trading up" at your dealership into a brand that has a better reputation for quality.