Measure your chain, if it has more than 1,500 miles it's probably worn/stretched.
To measure chain- take a tape or rule measuring from center of one link pin to the center of another pin 12 inches away while putting tension on the chain, if the mark isn't center of pin to center of pin and is center of one pin to the edge of the other pin or worse the chain is worn anshould be replaced, to simplify the procedure use a "chain gauge" Park makes a very good one.
Cogs- Look at your cogs (edge on) if the teeth on the cogs you are using most are thicker than the ones you use less, then the thick cogs are probably worn, also if you look at the cogs from the side and some are getting a "shark fin" profile they're probably worn. It'a judgment call, the gauges for cog wear aren't as available or accurate/easy to use as the chain gauge.
If your cogs/rings are very worn a new chain will grind or skip. So either use the old stuff until everything quits shifting/working right and replace the drive train or replace the chain and the indiviual cogs now, some makers only allow replacement of the entire cassette, guees who- S*******.
Ride with little wear
Pat