I have a Trane heat pump system XL 16i, outdoor model# 4TWZX6036B1000AA indoor model# 4TEE3F37B1000AA controlled with Honeywell VisionPro IAQ (YTH94Z1C1002) thermostat. The backup is electric strips.

I've had the system since 2007 and it's worked pretty well so far. However, today, I noticed a strange thing: the heat pump has been running all morning (from about 6:00am and it's about noon now), yet the temperature dropped from 69 to 66F. The thermostat is set to 70F and outside temperature currently is fairly mild 53F.
In the past, I experienced the situation where the heat pump couldn't keep the set temperature by itself but never when it was in the 50's outside. (the last time it happened the outside temps. was definitely below 30F).

I figured that maybe the filters were too dirty (although I clean them regularly and they didn't need to be cleaned yet per the indicator) so I cleaned them about 2 hours ago. That didn't make any difference - the heat pump keeps on running and the temp. sits at 66F.

Does anyone have an idea about what the problem may be?

Thanks.