A veteran economic forecaster and self-described former super bull is telling investors not to get too comfortable with the stock market's latest rally.

Economic Cycle Research Institute co-founder Lakshman Achuthan warned on CNBC's "Trading Nation" that an economic slowdown is already here — suggesting that Wall Street is dangerously optimistic right now.

"There's some tension here between what's going on with the economy outside your window and what's going on with expectations on Wall Street," Achuthan said Monday. "There is a stealth slowdown already happening."

He provided a chart that showed real consumer spending and income growth "rolling over."

"Since late last year, consumer spending growth has been easing, as has personal income growth," he noted. "That points to a fresh softening in a big part of the U.S. economy."

That's not the only bearish trend in the chart, according to Achuthan.

"Spending is outpacing income growth," he said. "That puts a lot of pressure on consumers, especially if interest rates are starting to rise."

His thoughts came as the Dow was rallying to close above the 25,000 mark for the first time since March. The index is now positive for the year. The also had a strong day, settling up 0.74 percent on Monday for its best day since May 10.

Achuthan wasn't sure when Wall Street could begin taking the economic sluggishness more seriously. But he still concluded that stocks are "cruising for a bruising right here."

"From an economic cycle risk point of view, the risk is rising," Achuthan said. "We have further slowing to go."